diff --git a/core/modules/aggregator/migrations/d6_aggregator_feed.yml b/core/modules/aggregator/migrations/d6_aggregator_feed.yml
index cad155374a..8689d5bcc8 100644
--- a/core/modules/aggregator/migrations/d6_aggregator_feed.yml
+++ b/core/modules/aggregator/migrations/d6_aggregator_feed.yml
@@ -1,5 +1,6 @@
 id: d6_aggregator_feed
 label: Aggregator feeds
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/aggregator/migrations/d6_aggregator_item.yml b/core/modules/aggregator/migrations/d6_aggregator_item.yml
index e14dbd60ed..7c991eb761 100644
--- a/core/modules/aggregator/migrations/d6_aggregator_item.yml
+++ b/core/modules/aggregator/migrations/d6_aggregator_item.yml
@@ -1,5 +1,6 @@
 id: d6_aggregator_item
 label: Aggregator items
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/aggregator/migrations/d7_aggregator_feed.yml b/core/modules/aggregator/migrations/d7_aggregator_feed.yml
index 5dbeb25eaf..48eb29de8c 100644
--- a/core/modules/aggregator/migrations/d7_aggregator_feed.yml
+++ b/core/modules/aggregator/migrations/d7_aggregator_feed.yml
@@ -1,5 +1,6 @@
 id: d7_aggregator_feed
 label: Aggregator feeds
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/aggregator/migrations/d7_aggregator_item.yml b/core/modules/aggregator/migrations/d7_aggregator_item.yml
index 054ba439f5..342c5c8cbb 100644
--- a/core/modules/aggregator/migrations/d7_aggregator_item.yml
+++ b/core/modules/aggregator/migrations/d7_aggregator_item.yml
@@ -1,5 +1,6 @@
 id: d7_aggregator_item
 label: Aggregator items
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/block_content/migrations/d6_custom_block.yml b/core/modules/block_content/migrations/d6_custom_block.yml
index 55fbcb5c9d..071e4de19a 100644
--- a/core/modules/block_content/migrations/d6_custom_block.yml
+++ b/core/modules/block_content/migrations/d6_custom_block.yml
@@ -1,5 +1,6 @@
 id: d6_custom_block
 label: Custom blocks
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/block_content/migrations/d7_custom_block.yml b/core/modules/block_content/migrations/d7_custom_block.yml
index ca06cf04f9..1a9ea1978c 100644
--- a/core/modules/block_content/migrations/d7_custom_block.yml
+++ b/core/modules/block_content/migrations/d7_custom_block.yml
@@ -1,5 +1,6 @@
 id: d7_custom_block
 label: Custom blocks
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/comment/migrations/d6_comment.yml b/core/modules/comment/migrations/d6_comment.yml
index 161820eeaa..afab0cb4ca 100644
--- a/core/modules/comment/migrations/d6_comment.yml
+++ b/core/modules/comment/migrations/d6_comment.yml
@@ -1,5 +1,6 @@
 id: d6_comment
 label: Comments
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/comment/migrations/d7_comment.yml b/core/modules/comment/migrations/d7_comment.yml
index dff4b64ab9..0837df91b7 100644
--- a/core/modules/comment/migrations/d7_comment.yml
+++ b/core/modules/comment/migrations/d7_comment.yml
@@ -1,5 +1,6 @@
 id: d7_comment
 label: Comments
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/file/migrations/d6_file.yml b/core/modules/file/migrations/d6_file.yml
index 6544d7d2f6..5b8b672018 100644
--- a/core/modules/file/migrations/d6_file.yml
+++ b/core/modules/file/migrations/d6_file.yml
@@ -2,6 +2,7 @@
 # migration as an optional dependency.
 id: d6_file
 label: Public files
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/file/migrations/d7_file.yml b/core/modules/file/migrations/d7_file.yml
index b63f13e9e0..7e05a28aeb 100644
--- a/core/modules/file/migrations/d7_file.yml
+++ b/core/modules/file/migrations/d7_file.yml
@@ -2,6 +2,7 @@
 # migration as an optional dependency.
 id: d7_file
 label: Public files
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/file/migrations/d7_file_private.yml b/core/modules/file/migrations/d7_file_private.yml
index 197c701031..51de533867 100644
--- a/core/modules/file/migrations/d7_file_private.yml
+++ b/core/modules/file/migrations/d7_file_private.yml
@@ -1,5 +1,6 @@
 id: d7_file_private
 label: Private files
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/menu_link_content/migrations/d6_menu_links.yml b/core/modules/menu_link_content/migrations/d6_menu_links.yml
index 2c8ad4a45a..e05efee4a7 100644
--- a/core/modules/menu_link_content/migrations/d6_menu_links.yml
+++ b/core/modules/menu_link_content/migrations/d6_menu_links.yml
@@ -1,5 +1,6 @@
 id: d6_menu_links
 label: Menu links
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/menu_link_content/migrations/d7_menu_links.yml b/core/modules/menu_link_content/migrations/d7_menu_links.yml
index 200a792047..81d4eb8530 100644
--- a/core/modules/menu_link_content/migrations/d7_menu_links.yml
+++ b/core/modules/menu_link_content/migrations/d7_menu_links.yml
@@ -1,5 +1,6 @@
 id: d7_menu_links
 label: Menu links
+audit: true
 migration_tags:
   - Drupal 7
 source:
diff --git a/core/modules/migrate/src/Audit/AuditException.php b/core/modules/migrate/src/Audit/AuditException.php
new file mode 100644
index 0000000000..3d122e30d8
--- /dev/null
+++ b/core/modules/migrate/src/Audit/AuditException.php
@@ -0,0 +1,27 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+use Drupal\migrate\Plugin\MigrationInterface;
+
+/**
+ * Exception thrown when a migration audit fails.
+ */
+class AuditException extends \RuntimeException {
+
+  /**
+   * AuditException constructor.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The migration that caused the exception.
+   * @param string $message
+   *   The reason the audit failed.
+   * @param \Exception $previous
+   *   (optional) The previous exception.
+   */
+  public function __construct(MigrationInterface $migration, $message, \Exception $previous = NULL) {
+    $message = sprintf('Cannot audit migration %s: %s', $migration->id(), $message);
+    parent::__construct($message, 0, $previous);
+  }
+
+}
diff --git a/core/modules/migrate/src/Audit/AuditResult.php b/core/modules/migrate/src/Audit/AuditResult.php
new file mode 100644
index 0000000000..d5260775e1
--- /dev/null
+++ b/core/modules/migrate/src/Audit/AuditResult.php
@@ -0,0 +1,146 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+use Drupal\Component\Render\MarkupInterface;
+use Drupal\migrate\Plugin\MigrationInterface;
+
+/**
+ * Encapsulates the result of a migration audit.
+ */
+class AuditResult implements MarkupInterface, \Countable {
+
+  /**
+   * The audited migration.
+   *
+   * @var \Drupal\migrate\Plugin\MigrationInterface
+   */
+  protected $migration;
+
+  /**
+   * The result of the audit (TRUE if passed, FALSE otherwise).
+   *
+   * @var bool
+   */
+  protected $status;
+
+  /**
+   * The reasons why the migration passed or failed the audit.
+   *
+   * @var string[]
+   */
+  protected $reasons = [];
+
+  /**
+   * AuditResult constructor.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The audited migration.
+   * @param bool $status
+   *   The result of the audit (TRUE if passed, FALSE otherwise).
+   * @param string[] $reasons
+   *   (optional) The reasons why the migration passed or failed the audit.
+   */
+  public function __construct(MigrationInterface $migration, $status, array $reasons = []) {
+    if (!is_bool($status)) {
+      throw new \InvalidArgumentException('Audit results must have a boolean status.');
+    }
+    $this->migration = $migration;
+    $this->status = $status;
+    array_walk($reasons, [$this, 'addReason']);
+  }
+
+  /**
+   * Returns the audited migration.
+   *
+   * @return \Drupal\migrate\Plugin\MigrationInterface
+   *   The audited migration.
+   */
+  public function getMigration() {
+    return $this->migration;
+  }
+
+  /**
+   * Returns the boolean result of the audit.
+   *
+   * @return bool
+   *   The result of the audit. TRUE if the migration passed the audit, FALSE
+   *   otherwise.
+   */
+  public function passed() {
+    return $this->status;
+  }
+
+  /**
+   * Adds a reason why the migration passed or failed the audit.
+   *
+   * @param string|object $reason
+   *   The reason to add. Can be a string or a string-castable object.
+   *
+   * @return $this
+   */
+  public function addReason($reason) {
+    array_push($this->reasons, (string) $reason);
+    return $this;
+  }
+
+  /**
+   * Creates a passing audit result for a migration.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The audited migration.
+   * @param string[] $reasons
+   *   (optional) The reasons why the migration passed the audit.
+   *
+   * @return static
+   */
+  public static function pass(MigrationInterface $migration, array $reasons = []) {
+    return new static($migration, TRUE, $reasons);
+  }
+
+  /**
+   * Creates a failing audit result for a migration.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The audited migration.
+   * @param array $reasons
+   *   (optional) The reasons why the migration failed the audit.
+   *
+   * @return static
+   */
+  public static function fail(MigrationInterface $migration, array $reasons = []) {
+    return new static($migration, FALSE, $reasons);
+  }
+
+  /**
+   * Implements \Countable::count() for Twig template compatibility.
+   *
+   * @return int
+   *
+   * @see \Drupal\Component\Render\MarkupInterface
+   */
+  public function count() {
+    return count($this->reasons);
+  }
+
+  /**
+   * Returns the reasons the migration passed or failed, as a string.
+   *
+   * @return string
+   *
+   * @see \Drupal\Component\Render\MarkupInterface
+   */
+  public function __toString() {
+    return implode("\n", $this->reasons);
+  }
+
+  /**
+   * Returns the reasons the migration passed or failed, for JSON serialization.
+   *
+   * @return string[]
+   */
+  public function jsonSerialize() {
+    return $this->reasons;
+  }
+
+}
diff --git a/core/modules/migrate/src/Audit/AuditorInterface.php b/core/modules/migrate/src/Audit/AuditorInterface.php
new file mode 100644
index 0000000000..a61b792cfe
--- /dev/null
+++ b/core/modules/migrate/src/Audit/AuditorInterface.php
@@ -0,0 +1,42 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+use Drupal\migrate\Plugin\MigrationInterface;
+
+/**
+ * Defines an interface for migration auditors.
+ *
+ * A migration auditor is a class which can examine a migration to determine if
+ * it will cause conflicts with data already existing in the destination system.
+ * What kind of auditing it does, and how it does it, is up to the implementing
+ * class.
+ */
+interface AuditorInterface {
+
+  /**
+   * Audits a migration.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface $migration
+   *   The migration to audit.
+   *
+   * @throws \Drupal\migrate\Audit\AuditException
+   *   If the audit fails.
+   *
+   * @return \Drupal\migrate\Audit\AuditResult
+   *   The result of the audit.
+   */
+  public function audit(MigrationInterface $migration);
+
+  /**
+   * Audits a set of migrations.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface[] $migrations
+   *   The migrations to audit.
+   *
+   * @return \Drupal\migrate\Audit\AuditResult[]
+   *   The audit results, keyed by migration ID.
+   */
+  public function auditMultiple(array $migrations);
+
+}
diff --git a/core/modules/migrate/src/Audit/IdAuditor.php b/core/modules/migrate/src/Audit/IdAuditor.php
new file mode 100644
index 0000000000..c4b4096708
--- /dev/null
+++ b/core/modules/migrate/src/Audit/IdAuditor.php
@@ -0,0 +1,60 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+use Drupal\Core\StringTranslation\StringTranslationTrait;
+use Drupal\migrate\Plugin\MigrationInterface;
+
+/**
+ * Audits migrations that create content entities in the destination system.
+ */
+class IdAuditor implements AuditorInterface {
+
+  use StringTranslationTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  public function audit(MigrationInterface $migration) {
+    $plugin_definition = $migration->getPluginDefinition();
+
+    // If the migration does not opt into auditing, it passes.
+    if (empty($plugin_definition['audit'])) {
+      return AuditResult::pass($migration);
+    }
+
+    $interface = MaximumValueInterface::class;
+
+    $destination = $migration->getDestinationPlugin();
+    if (!$destination instanceof MaximumValueInterface) {
+      throw new AuditException($migration, "Destination does not implement $interface");
+    }
+
+    $id_map = $migration->getIdMap();
+    if (!$id_map instanceof MaximumValueInterface) {
+      throw new AuditException($migration, "ID map does not implement $interface");
+    }
+
+    if ($destination->getHighestId() > $id_map->getHighestId()) {
+      return AuditResult::fail($migration, [
+        $this->t('The destination system contains data which was not created by a migration.'),
+      ]);
+    }
+    return AuditResult::pass($migration);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function auditMultiple(array $migrations) {
+    $conflicts = [];
+
+    foreach ($migrations as $migration) {
+      $migration_id = $migration->getPluginId();
+      $conflicts[$migration_id] = $this->audit($migration);
+    }
+    ksort($conflicts);
+    return $conflicts;
+  }
+
+}
diff --git a/core/modules/migrate/src/Audit/MaximumValueInterface.php b/core/modules/migrate/src/Audit/MaximumValueInterface.php
new file mode 100644
index 0000000000..fb84d2b73d
--- /dev/null
+++ b/core/modules/migrate/src/Audit/MaximumValueInterface.php
@@ -0,0 +1,26 @@
+<?php
+
+namespace Drupal\migrate\Audit;
+
+/**
+ * Defines an interface for destination and ID maps which track a highest ID.
+ *
+ * When implemented by destination plugins, getHighestId() should return the
+ * highest ID of the destination entity type that exists in the system. So, for
+ * example, the entity:node plugin should return the highest node ID that
+ * exists, regardless of whether it was created by a migration.
+ *
+ * When implemented by an ID map, getHighestId() should return the highest
+ * migrated ID of the destination entity type.
+ */
+interface MaximumValueInterface {
+
+  /**
+   * Returns the highest ID value tracked by the implementing plugin.
+   *
+   * @return int
+   *   The highest ID.
+   */
+  public function getHighestId();
+
+}
diff --git a/core/modules/migrate/src/Plugin/Migration.php b/core/modules/migrate/src/Plugin/Migration.php
index c81d498460..ba55ed9361 100644
--- a/core/modules/migrate/src/Plugin/Migration.php
+++ b/core/modules/migrate/src/Plugin/Migration.php
@@ -16,6 +16,12 @@
  * The migration process plugin represents one single migration and acts like a
  * container for the information about a single migration such as the source,
  * process and destination plugins.
+ *
+ * Configuration options:
+ *
+ *   - audit: If set to TRUE, the migration's IDs will be audited. This means
+ *     that if the highest destination ID is greater than the highest source ID,
+ *     a warning will be displayed that entities might be overwritten.
  */
 class Migration extends PluginBase implements MigrationInterface, RequirementsInterface, ContainerFactoryPluginInterface {
 
diff --git a/core/modules/migrate/src/Plugin/migrate/destination/Entity.php b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
index 5d40593962..b83f712519 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/Entity.php
@@ -94,6 +94,10 @@
    *   The list of bundles this entity type has.
    */
   public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, EntityStorageInterface $storage, array $bundles) {
+    $plugin_definition += [
+      'label' => $storage->getEntityType()->getPluralLabel(),
+    ];
+
     parent::__construct($configuration, $plugin_id, $plugin_definition, $migration);
     $this->storage = $storage;
     $this->bundles = $bundles;
diff --git a/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
index cf3ae1dedb..041443f476 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
@@ -9,6 +9,7 @@
 use Drupal\Core\Field\FieldTypePluginManagerInterface;
 use Drupal\Core\TypedData\TranslatableInterface;
 use Drupal\Core\TypedData\TypedDataInterface;
+use Drupal\migrate\Audit\MaximumValueInterface;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\MigrateException;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
@@ -18,7 +19,7 @@
 /**
  * The destination class for all content entities lacking a specific class.
  */
-class EntityContentBase extends Entity {
+class EntityContentBase extends Entity implements MaximumValueInterface {
 
   /**
    * Entity manager.
@@ -111,12 +112,9 @@ protected function save(ContentEntityInterface $entity, array $old_destination_i
   }
 
   /**
-   * Get whether this destination is for translations.
-   *
-   * @return bool
-   *   Whether this destination is for translations.
+   * {@inheritdoc}
    */
-  protected function isTranslationDestination() {
+  public function isTranslationDestination() {
     return !empty($this->configuration['translations']);
   }
 
@@ -294,4 +292,15 @@ protected function getDefinitionFromEntity($key) {
     ] + $field_definition->getSettings();
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getHighestId() {
+    $query = $this->storage->getQuery()
+      ->sort($this->getKey('id'), 'DESC')
+      ->range(0, 1);
+    $result = $query->execute();
+    return (int) reset($result);
+  }
+
 }
diff --git a/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
index b0db476987..f617c7756c 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
@@ -78,4 +78,15 @@ public function getIds() {
     throw new MigrateException('This entity type does not support revisions.');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getHighestId() {
+    $query = $this->storage->getQuery()
+      ->sort($this->getKey('revision'), 'DESC')
+      ->range(0, 1);
+    $result = $query->execute();
+    return (int) reset($result);
+  }
+
 }
diff --git a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
index 6bdd51ebf0..178d316e99 100644
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -7,6 +7,7 @@
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Plugin\PluginBase;
 use Drupal\migrate\MigrateMessage;
+use Drupal\migrate\Audit\MaximumValueInterface;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\Event\MigrateIdMapMessageEvent;
 use Drupal\migrate\MigrateException;
@@ -27,7 +28,7 @@
  *
  * @PluginID("sql")
  */
-class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryPluginInterface {
+class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryPluginInterface, MaximumValueInterface {
 
   /**
    * Column name of hashed source id values.
@@ -152,6 +153,8 @@ class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryP
    *   The configuration for the plugin.
    * @param \Drupal\migrate\Plugin\MigrationInterface $migration
    *   The migration to do.
+   * @param \Symfony\Component\EventDispatcher\EventDispatcherInterface $event_dispatcher
+   *   The event dispatcher.
    */
   public function __construct(array $configuration, $plugin_id, $plugin_definition, MigrationInterface $migration, EventDispatcherInterface $event_dispatcher) {
     parent::__construct($configuration, $plugin_id, $plugin_definition);
@@ -169,7 +172,8 @@ public static function create(ContainerInterface $container, array $configuratio
       $plugin_id,
       $plugin_definition,
       $migration,
-      $container->get('event_dispatcher')
+      $container->get('event_dispatcher'),
+      $container->get('plugin.manager.migration')
     );
   }
 
@@ -925,4 +929,69 @@ public function valid() {
     return $this->currentRow !== FALSE;
   }
 
+  /**
+   * Get the migration plugin manager.
+   *
+   * @todo Inject as a dependency in https://www.drupal.org/node/2919158.
+   *
+   * @return \Drupal\migrate\Plugin\MigrationPluginManagerInterface
+   *   The migration plugin manager.
+   */
+  protected function getMigrationPluginManager() {
+    return \Drupal::service('plugin.manager.migration');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getHighestId() {
+    array_filter(
+      $this->migration->getDestinationPlugin()->getIds(),
+      function (array $id) {
+        if ($id['type'] !== 'integer') {
+          throw new \LogicException('Cannot determine the highest migrated ID without an integer ID column');
+        }
+      }
+    );
+
+    // List of mapping tables to look in for the highest ID.
+    $map_tables = [
+      $this->migration->id() => $this->mapTableName(),
+    ];
+
+    // If there's a bundle, it means we have a derived migration and we need to
+    // find all the mapping tables from the related derived migrations.
+    if ($base_id = substr($this->migration->id(), 0, strpos($this->migration->id(), static::DERIVATIVE_SEPARATOR))) {
+      $migration_manager = $this->getMigrationPluginManager();
+      $migrations = $migration_manager->getDefinitions();
+      foreach ($migrations as $migration_id => $migration) {
+        if ($migration['id'] === $base_id) {
+          // Get this derived migration's mapping table and add it to the list
+          // of mapping tables to look in for the highest ID.
+          $stub = $migration_manager->createInstance($migration_id);
+          $map_tables[$migration_id] = $stub->getIdMap()->mapTableName();
+        }
+      }
+    }
+
+    // Get the highest id from the list of map tables.
+    $ids = [0];
+    foreach ($map_tables as $map_table) {
+      if (!$this->getDatabase()->schema()->tableExists($map_table)) {
+        break;
+      }
+
+      $query = $this->getDatabase()->select($map_table, 'map')
+        ->fields('map', $this->destinationIdFields())
+        ->range(0, 1);
+      foreach (array_values($this->destinationIdFields()) as $order_field) {
+        $query->orderBy($order_field, 'DESC');
+      }
+      $ids[] = $query->execute()->fetchField();
+    }
+
+    // Return the highest of all the mapped IDs.
+    return max($ids);
+  }
+
 }
diff --git a/core/modules/migrate/tests/src/Unit/Plugin/migrate/destination/EntityContentBaseTest.php b/core/modules/migrate/tests/src/Unit/Plugin/migrate/destination/EntityContentBaseTest.php
index 4fa08fed0a..c0f7f19743 100644
--- a/core/modules/migrate/tests/src/Unit/Plugin/migrate/destination/EntityContentBaseTest.php
+++ b/core/modules/migrate/tests/src/Unit/Plugin/migrate/destination/EntityContentBaseTest.php
@@ -11,6 +11,7 @@
 use Drupal\Core\Entity\ContentEntityType;
 use Drupal\Core\Entity\EntityManagerInterface;
 use Drupal\Core\Entity\EntityStorageInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\Core\Field\BaseFieldDefinition;
 use Drupal\Core\Field\FieldTypePluginManagerInterface;
 use Drupal\migrate\MigrateException;
@@ -39,6 +40,11 @@ class EntityContentBaseTest extends UnitTestCase {
   protected $storage;
 
   /**
+   * @var \Drupal\Core\Entity\EntityTypeInterface
+   */
+  protected $entityType;
+
+  /**
    * @var \Drupal\Core\Entity\EntityManagerInterface
    */
   protected $entityManager;
@@ -51,6 +57,11 @@ protected function setUp() {
 
     $this->migration = $this->prophesize(MigrationInterface::class);
     $this->storage = $this->prophesize(EntityStorageInterface::class);
+
+    $this->entityType = $this->prophesize(EntityTypeInterface::class);
+    $this->entityType->getPluralLabel()->willReturn('wonkiness');
+    $this->storage->getEntityType()->willReturn($this->entityType->reveal());
+
     $this->entityManager = $this->prophesize(EntityManagerInterface::class);
   }
 
@@ -104,14 +115,11 @@ public function testImportEntityLoadFailure() {
    */
   public function testUntranslatable() {
     // An entity type without a language.
-    $entity_type = $this->prophesize(ContentEntityType::class);
-    $entity_type->getKey('langcode')->willReturn('');
-    $entity_type->getKey('id')->willReturn('id');
+    $this->entityType->getKey('langcode')->willReturn('');
+    $this->entityType->getKey('id')->willReturn('id');
     $this->entityManager->getBaseFieldDefinitions('foo')
       ->willReturn(['id' => BaseFieldDefinitionTest::create('integer')]);
 
-    $this->storage->getEntityType()->willReturn($entity_type->reveal());
-
     $destination = new EntityTestDestination(
       ['translations' => TRUE],
       '',
diff --git a/core/modules/migrate/tests/src/Unit/destination/EntityRevisionTest.php b/core/modules/migrate/tests/src/Unit/destination/EntityRevisionTest.php
index f68c2bea1f..489a7b8bd3 100644
--- a/core/modules/migrate/tests/src/Unit/destination/EntityRevisionTest.php
+++ b/core/modules/migrate/tests/src/Unit/destination/EntityRevisionTest.php
@@ -9,6 +9,7 @@
 
 use Drupal\Core\Entity\ContentEntityInterface;
 use Drupal\Core\Entity\EntityInterface;
+use Drupal\Core\Entity\EntityTypeInterface;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\Plugin\migrate\destination\EntityRevision as RealEntityRevision;
 use Drupal\migrate\Row;
@@ -48,6 +49,11 @@ protected function setUp() {
     // Setup mocks to be used when creating a revision destination.
     $this->migration = $this->prophesize(MigrationInterface::class);
     $this->storage = $this->prophesize('\Drupal\Core\Entity\EntityStorageInterface');
+
+    $entity_type = $this->prophesize(EntityTypeInterface::class);
+    $entity_type->getPluralLabel()->willReturn('craziness');
+    $this->storage->getEntityType()->willReturn($entity_type->reveal());
+
     $this->entityManager = $this->prophesize('\Drupal\Core\Entity\EntityManagerInterface');
     $this->fieldTypeManager = $this->prophesize('\Drupal\Core\Field\FieldTypePluginManagerInterface');
   }
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/CreateContentTrait.php b/core/modules/migrate_drupal/tests/src/Kernel/CreateContentTrait.php
new file mode 100644
index 0000000000..9cf22c6723
--- /dev/null
+++ b/core/modules/migrate_drupal/tests/src/Kernel/CreateContentTrait.php
@@ -0,0 +1,92 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal\Kernel;
+
+use Drupal\aggregator\Entity\Feed;
+use Drupal\aggregator\Entity\Item;
+use Drupal\block_content\Entity\BlockContent;
+use Drupal\comment\Entity\Comment;
+use Drupal\file\Entity\File;
+use Drupal\menu_link_content\Entity\MenuLinkContent;
+use Drupal\node\Entity\Node;
+use Drupal\taxonomy\Entity\Term;
+use Drupal\user\Entity\User;
+
+/**
+ * Provides helper methods for creating content.
+ */
+trait CreateContentTrait {
+
+  /**
+   * Create several pieces of generic content.
+   */
+  protected function createContent() {
+    // Create an aggregator feed.
+    $feed = Feed::create([
+      'title' => 'feed',
+      'url' => 'http://www.example.com',
+    ]);
+    $feed->save();
+
+    // Create an aggregator feed item.
+    $item = Item::create([
+      'title' => 'feed item',
+      'fid' => $feed->id(),
+      'link' => 'http://www.example.com',
+    ]);
+    $item->save();
+
+    // Create a block content.
+    $block = BlockContent::create([
+      'info' => 'block',
+      'type' => 'block',
+    ]);
+    $block->save();
+
+    // Create a node.
+    $node = Node::create([
+      'type' => 'page',
+      'title' => 'page',
+    ]);
+    $node->save();
+
+    // Create a comment.
+    $comment = Comment::create([
+      'comment_type' => 'comment',
+      'field_name' => 'comment',
+      'entity_type' => 'node',
+      'entity_id' => $node->id(),
+    ]);
+    $comment->save();
+
+    // Create a file.
+    $file = File::create([
+      'uri' => 'public://example.txt',
+    ]);
+    $file->save();
+
+    // Create a menu link.
+    $menu_link = MenuLinkContent::create([
+      'title' => 'menu link',
+      'link' => ['uri' => 'http://www.example.com'],
+      'menu_name' => 'tools',
+    ]);
+    $menu_link->save();
+
+    // Create a taxonomy term.
+    $term = Term::create([
+      'name' => 'term',
+      'vid' => 'term',
+    ]);
+    $term->save();
+
+    // Create a user.
+    $user = User::create([
+      'uid' => 2,
+      'name' => 'user',
+      'mail' => 'user@example.com',
+    ]);
+    $user->save();
+  }
+
+}
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/MigrateDrupalTestBase.php b/core/modules/migrate_drupal/tests/src/Kernel/MigrateDrupalTestBase.php
index 063c99cb36..987c05013e 100644
--- a/core/modules/migrate_drupal/tests/src/Kernel/MigrateDrupalTestBase.php
+++ b/core/modules/migrate_drupal/tests/src/Kernel/MigrateDrupalTestBase.php
@@ -3,6 +3,8 @@
 namespace Drupal\Tests\migrate_drupal\Kernel;
 
 use Drupal\Core\Database\Database;
+use Drupal\migrate\Audit\AuditResult;
+use Drupal\migrate\Audit\IdAuditor;
 use Drupal\Tests\migrate\Kernel\MigrateTestBase;
 
 /**
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php b/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php
new file mode 100644
index 0000000000..afb94aed97
--- /dev/null
+++ b/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php
@@ -0,0 +1,136 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal\Kernel\d6;
+
+use Drupal\KernelTests\FileSystemModuleDiscoveryDataProviderTrait;
+use Drupal\migrate\Audit\AuditResult;
+use Drupal\migrate\Audit\IdAuditor;
+use Drupal\node\Entity\Node;
+use Drupal\Tests\migrate_drupal\Kernel\CreateContentTrait;
+
+/**
+ * Tests the migration auditor for ID conflicts.
+ *
+ * @group migrate_drupal
+ */
+class MigrateDrupal6AuditIdsTest extends MigrateDrupal6TestBase {
+
+  use FileSystemModuleDiscoveryDataProviderTrait;
+  use CreateContentTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    // Enable all modules.
+    self::$modules = array_keys($this->coreModuleListDataProvider());
+    parent::setUp();
+
+    // Install required schemas.
+    $this->installEntitySchema('aggregator_item');
+    $this->installEntitySchema('aggregator_feed');
+    $this->installEntitySchema('block_content');
+    $this->installEntitySchema('comment');
+    $this->installEntitySchema('file');
+    $this->installEntitySchema('menu_link_content');
+    $this->installEntitySchema('node');
+    $this->installEntitySchema('taxonomy_term');
+    $this->installSchema('book', ['book']);
+    $this->installSchema('dblog', ['watchdog']);
+    $this->installSchema('forum', ['forum_index']);
+    $this->installSchema('search', ['search_dataset']);
+    $this->installSchema('system', ['sequences']);
+    $this->installSchema('tracker', ['tracker_node', 'tracker_user']);
+  }
+
+  /**
+   * Tests multiple migrations to the same destination with no ID conflicts.
+   */
+  public function testMultipleMigrationWithoutIdConflicts() {
+    // Create a node of type page.
+    Node::create(['type' => 'page', 'title' => 'foo'])->save();
+
+    // Insert data in the d6_node:page migration mappping table to simulate a
+    // previously migrated node.
+    $table_name = $this->getMigration('d6_node:page')->getIdMap()->mapTableName();
+    $this->container->get('database')->insert($table_name)
+      ->fields([
+        'source_ids_hash' => 1,
+        'sourceid1' => 1,
+        'destid1' => 1,
+      ])
+      ->execute();
+
+    // Audit the IDs of the d6_node migrations for the page & article node type.
+    // There should be no conflicts since the highest destination ID should be
+    // equal to the highest migrated ID, as found in the aggregated mapping
+    // tables of the two node migrations.
+    $migrations = [
+      $this->getMigration('d6_node:page'),
+      $this->getMigration('d6_node:article'),
+    ];
+
+    $results = (new IdAuditor())->auditMultiple($migrations);
+    /** @var \Drupal\migrate\Audit\AuditResult $result */
+    foreach ($results as $result) {
+      $this->assertInstanceOf(AuditResult::class, $result);
+      $this->assertTrue($result->passed());
+    }
+  }
+
+  /**
+   * Tests all migrations with no ID conflicts.
+   */
+  public function testAllMigrationsWithNoIdConflicts() {
+    $migrations = $this->container
+      ->get('plugin.manager.migration')
+      ->createInstancesByTag('Drupal 6');
+
+    // Audit all Drupal 6 migrations that support it. There should be no
+    // conflicts since no content has been created.
+    $results = (new IdAuditor())->auditMultiple($migrations);
+    /** @var \Drupal\migrate\Audit\AuditResult $result */
+    foreach ($results as $result) {
+      $this->assertInstanceOf(AuditResult::class, $result);
+      $this->assertTrue($result->passed());
+    }
+  }
+
+  /**
+   * Tests all migrations with ID conflicts.
+   */
+  public function testAllMigrationsWithIdConflicts() {
+    // Get all Drupal 6 migrations.
+    $migrations = $this->container
+      ->get('plugin.manager.migration')
+      ->createInstancesByTag('Drupal 6');
+
+    // Create content.
+    $this->createContent();
+
+    // Audit the IDs of all migrations. There should be conflicts since content
+    // has been created.
+    $conflicts = array_map(
+      function (AuditResult $result) {
+        return $result->passed() ? NULL : $result->getMigration()->getBaseId();
+      },
+      (new IdAuditor())->auditMultiple($migrations)
+    );
+
+    $expected = [
+      'd6_aggregator_feed',
+      'd6_aggregator_item',
+      'd6_comment',
+      'd6_custom_block',
+      'd6_file',
+      'd6_menu_links',
+      'd6_node',
+      'd6_node_revision',
+      'd6_taxonomy_term',
+      'd6_term_node_revision',
+      'd6_user',
+    ];
+    $this->assertEmpty(array_diff(array_filter($conflicts), $expected));
+  }
+
+}
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php b/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php
new file mode 100644
index 0000000000..39b59aa9ad
--- /dev/null
+++ b/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php
@@ -0,0 +1,135 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal\Kernel\d7;
+
+use Drupal\KernelTests\FileSystemModuleDiscoveryDataProviderTrait;
+use Drupal\migrate\Audit\AuditResult;
+use Drupal\migrate\Audit\IdAuditor;
+use Drupal\node\Entity\Node;
+use Drupal\Tests\migrate_drupal\Kernel\CreateContentTrait;
+
+/**
+ * Tests the migration auditor for ID conflicts.
+ *
+ * @group migrate_drupal
+ */
+class MigrateDrupal7AuditIdsTest extends MigrateDrupal7TestBase {
+
+  use FileSystemModuleDiscoveryDataProviderTrait;
+  use CreateContentTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    // Enable all modules.
+    self::$modules = array_keys($this->coreModuleListDataProvider());
+    parent::setUp();
+
+    // Install required schemas.
+    $this->installEntitySchema('aggregator_item');
+    $this->installEntitySchema('aggregator_feed');
+    $this->installEntitySchema('block_content');
+    $this->installEntitySchema('comment');
+    $this->installEntitySchema('file');
+    $this->installEntitySchema('menu_link_content');
+    $this->installEntitySchema('node');
+    $this->installEntitySchema('taxonomy_term');
+    $this->installSchema('book', ['book']);
+    $this->installSchema('dblog', ['watchdog']);
+    $this->installSchema('forum', ['forum_index']);
+    $this->installSchema('search', ['search_dataset']);
+    $this->installSchema('system', ['sequences']);
+    $this->installSchema('tracker', ['tracker_node', 'tracker_user']);
+  }
+
+  /**
+   * Tests multiple migrations to the same destination with no ID conflicts.
+   */
+  public function testMultipleMigrationWithoutIdConflicts() {
+    // Create a node of type page.
+    Node::create(['type' => 'page', 'title' => 'foo'])->save();
+
+    // Insert data in the d7_node:page migration mappping table to simulate a
+    // previously migrated node.
+    $table_name = $this->getMigration('d7_node:page')->getIdMap()->mapTableName();
+    $this->container->get('database')->insert($table_name)
+      ->fields([
+        'source_ids_hash' => 1,
+        'sourceid1' => 1,
+        'destid1' => 1,
+      ])
+      ->execute();
+
+    // Audit the IDs of the d7_node migrations for the page & article node type.
+    // There should be no conflicts since the highest destination ID should be
+    // equal to the highest migrated ID, as found in the aggregated mapping
+    // tables of the two node migrations.
+    $migrations = [
+      $this->getMigration('d7_node:page'),
+      $this->getMigration('d7_node:article'),
+    ];
+
+    $results = (new IdAuditor())->auditMultiple($migrations);
+    /** @var \Drupal\migrate\Audit\AuditResult $result */
+    foreach ($results as $result) {
+      $this->assertInstanceOf(AuditResult::class, $result);
+      $this->assertTrue($result->passed());
+    }
+  }
+
+  /**
+   * Tests all migrations with no ID conflicts.
+   */
+  public function testAllMigrationsWithNoIdConflicts() {
+    $migrations = $this->container
+      ->get('plugin.manager.migration')
+      ->createInstancesByTag('Drupal 7');
+
+    // Audit the IDs of all Drupal 7 migrations. There should be no conflicts
+    // since no content has been created.
+    $results = (new IdAuditor())->auditMultiple($migrations);
+    /** @var \Drupal\migrate\Audit\AuditResult $result */
+    foreach ($results as $result) {
+      $this->assertInstanceOf(AuditResult::class, $result);
+      $this->assertTrue($result->passed());
+    }
+  }
+
+  /**
+   * Tests all migrations with ID conflicts.
+   */
+  public function testAllMigrationsWithIdConflicts() {
+    $migrations = $this->container
+      ->get('plugin.manager.migration')
+      ->createInstancesByTag('Drupal 7');
+
+    // Create content.
+    $this->createContent();
+
+    // Audit the IDs of all Drupal 7 migrations. There should be conflicts since
+    // content has been created.
+    $conflicts = array_map(
+      function (AuditResult $result) {
+        return $result->passed() ? NULL : $result->getMigration()->getBaseId();
+      },
+      (new IdAuditor())->auditMultiple($migrations)
+    );
+
+    $expected = [
+      'd7_aggregator_feed',
+      'd7_aggregator_item',
+      'd7_comment',
+      'd7_custom_block',
+      'd7_file',
+      'd7_file_private',
+      'd7_menu_links',
+      'd7_node',
+      'd7_node_revision',
+      'd7_taxonomy_term',
+      'd7_user',
+    ];
+    $this->assertEmpty(array_diff(array_filter($conflicts), $expected));
+  }
+
+}
diff --git a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
index 6bfaf42f1d..908b17ad7b 100644
--- a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
@@ -9,6 +9,8 @@
 use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\State\StateInterface;
 use Drupal\Core\Url;
+use Drupal\migrate\Audit\IdAuditor;
+use Drupal\migrate\Plugin\migrate\destination\EntityContentBase;
 use Drupal\migrate\Plugin\MigrationPluginManagerInterface;
 use Drupal\migrate_drupal\Plugin\MigrateFieldPluginManagerInterface;
 use Drupal\migrate_drupal_ui\Batch\MigrateUpgradeImportBatch;
@@ -67,6 +69,13 @@ class MigrateUpgradeForm extends ConfirmFormBase {
   protected $moduleHandler;
 
   /**
+   * The auditor service.
+   *
+   * @var \Drupal\migrate\Audit\AuditorInterface
+   */
+  protected $auditor;
+
+  /**
    * Constructs the MigrateUpgradeForm.
    *
    * @param \Drupal\Core\State\StateInterface $state
@@ -124,6 +133,9 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       case 'credentials':
         return $this->buildCredentialForm($form, $form_state);
 
+      case 'confirm_id_conflicts':
+        return $this->buildIdConflictForm($form, $form_state);
+
       case 'confirm':
         return $this->buildConfirmForm($form, $form_state);
 
@@ -457,6 +469,133 @@ public function validateCredentialForm(array &$form, FormStateInterface $form_st
    */
   public function submitCredentialForm(array &$form, FormStateInterface $form_state) {
     // Indicate the next step is confirmation.
+    $form_state->set('step', 'confirm_id_conflicts');
+    $form_state->setRebuild();
+  }
+
+  /**
+   * Confirmation form for ID conflicts.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   *
+   * @return array
+   *   The form structure.
+   */
+  public function buildIdConflictForm(array &$form, FormStateInterface $form_state) {
+    // Check if there are conflicts. If none, just skip this form!
+    $migration_ids = array_keys($form_state->get('migrations'));
+    $migrations = $this->pluginManager->createInstances($migration_ids);
+
+    $translated_content_conflicts = $content_conflicts = [];
+
+    $results = (new IdAuditor())->auditMultiple($migrations);
+
+    /** @var \Drupal\migrate\Audit\AuditResult $result */
+    foreach ($results as $migration_id => $result) {
+      // If the migration passed the audit, we don't need to do anything.
+      if ($result->passed()) {
+        continue;
+      }
+
+      $destination = $result->getMigration()->getDestinationPlugin();
+      $definition = $destination->getPluginDefinition();
+
+      if ($destination instanceof EntityContentBase && $destination->isTranslationDestination()) {
+        $translated_content_conflicts[$migration_id] = $definition['label'];
+      }
+      else {
+        $content_conflicts[$migration_id] = $definition['label'];
+      }
+    }
+    if (empty($content_conflicts) && empty($translated_content_conflicts)) {
+      $form_state->set('step', 'confirm');
+      return $this->buildForm($form, $form_state);
+    }
+
+    $form['warning'] = [
+      '#type' => 'markup',
+      '#markup' => '<h3>' . $this->t('Content may be overwritten') . '</h3>',
+    ];
+    $form = parent::buildForm($form, $form_state);
+    $form['actions']['submit']['#submit'] = ['::submitConfirmIdConflictForm'];
+    $form['actions']['submit']['#value'] = $this->t('I acknowledge I may lose data, continue anyway.');
+
+    $form = $this->conflictsForm($form, $form_state, $content_conflicts);
+    return $this->i18nWarningForm($form, $form_state, $translated_content_conflicts);
+  }
+
+  /**
+   * Build the markup for conflict warnings.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param array $content_conflicts
+   *   The conflicted migration labels.
+   *
+   * @return array
+   *   The form structure.
+   */
+  protected function conflictsForm(array &$form, FormStateInterface $form_state, $content_conflicts) {
+    if (empty($content_conflicts)) {
+      return $form;
+    }
+
+    $form['warning']['#markup'] .= '<p>' . $this->t('Upgrades work on clean and empty new installs of Drupal 8. However, it looks like you have content in your site that is unknown to the migrate system; perhaps manually added. These new entities <strong>may be overwritten</strong> if you run this upgrade. For more information, see the online handbook entry for <a target="_blank" href=":id-conflicts-handbook">handling migration conflicts</a>.', [':id-conflicts-handbook' => 'https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#id_conflicts']) . '</p>';
+
+    sort($content_conflicts);
+    $form['conflicts'] = [
+      '#title' => $this->t('The conflicting content are of the following types:'),
+      '#theme' => 'item_list',
+      '#items' => $content_conflicts,
+    ];
+
+    return $form;
+  }
+
+  /**
+   * Build the markup for i18n warnings.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   * @param array $translated_content_conflicts
+   *   The conflicted translated migration labels.
+   *
+   * @return array
+   *   The form structure.
+   */
+  protected function i18nWarningForm(array &$form, FormStateInterface $form_state, $translated_content_conflicts) {
+    if (empty($translated_content_conflicts)) {
+      return $form;
+    }
+
+    $form['warning']['#markup'] .= '<p>' . $this->t('It looks like you are migrating translated content. Be extra cautious and make sure you aren\'t introducting any conflicts. For more on the subject, see the online handbook entry for <a target="_blank" href=":id-conflicts-handbook">handling migration conflicts</a>.', [':id-conflicts-handbook' => 'https://www.drupal.org/docs/8/upgrade/known-issues-when-upgrading-from-drupal-6-or-7-to-drupal-8#id_conflicts']) . '</p>';
+
+    sort($translated_content_conflicts);
+    $form['i18n'] = [
+      '#title' => $this->t('The translated content are of the following types:'),
+      '#theme' => 'item_list',
+      '#items' => $translated_content_conflicts,
+    ];
+
+    return $form;
+  }
+
+  /**
+   * Submission handler for the confirmation form.
+   *
+   * @param array $form
+   *   An associative array containing the structure of the form.
+   * @param \Drupal\Core\Form\FormStateInterface $form_state
+   *   The current state of the form.
+   */
+  public function submitConfirmIdConflictForm(array &$form, FormStateInterface $form_state) {
     $form_state->set('step', 'confirm');
     $form_state->setRebuild();
   }
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
index 09dc9691e3..9581312aaa 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -151,6 +151,8 @@ public function testMigrateUpgrade() {
     $this->assertText('Resolve the issue below to continue the upgrade.');
 
     $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->assertSession()->pageTextContains('Content may be overwritten');
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
     $this->assertResponse(200);
     $this->assertText('Upgrade analysis report');
     // Ensure we get errors about missing modules.
@@ -170,6 +172,8 @@ public function testMigrateUpgrade() {
     $this->assertSession()->fieldExists('mysql[host]');
 
     $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->assertSession()->pageTextContains('Content may be overwritten');
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
     $this->assertSession()->statusCodeEquals(200);
     $this->assertSession()->pageTextContains('Upgrade analysis report');
     // Ensure there are no errors about the missing modules from the test module.
diff --git a/core/modules/node/migrations/d6_node.yml b/core/modules/node/migrations/d6_node.yml
index 56d0459a81..84a4bf1872 100644
--- a/core/modules/node/migrations/d6_node.yml
+++ b/core/modules/node/migrations/d6_node.yml
@@ -1,5 +1,6 @@
 id: d6_node
 label: Nodes
+audit: true
 migration_tags:
   - Drupal 6
 deriver: Drupal\node\Plugin\migrate\D6NodeDeriver
diff --git a/core/modules/node/migrations/d6_node_revision.yml b/core/modules/node/migrations/d6_node_revision.yml
index f4ff3011c4..74a42d4307 100644
--- a/core/modules/node/migrations/d6_node_revision.yml
+++ b/core/modules/node/migrations/d6_node_revision.yml
@@ -1,5 +1,6 @@
 id: d6_node_revision
 label: Node revisions
+audit: true
 migration_tags:
   - Drupal 6
 deriver: Drupal\node\Plugin\migrate\D6NodeDeriver
diff --git a/core/modules/node/migrations/d7_node.yml b/core/modules/node/migrations/d7_node.yml
index 359be81cc4..80367971a3 100644
--- a/core/modules/node/migrations/d7_node.yml
+++ b/core/modules/node/migrations/d7_node.yml
@@ -1,5 +1,6 @@
 id: d7_node
 label: Nodes
+audit: true
 migration_tags:
   - Drupal 7
 deriver: Drupal\node\Plugin\migrate\D7NodeDeriver
diff --git a/core/modules/node/migrations/d7_node_revision.yml b/core/modules/node/migrations/d7_node_revision.yml
index c6081ef110..18c90b6f49 100644
--- a/core/modules/node/migrations/d7_node_revision.yml
+++ b/core/modules/node/migrations/d7_node_revision.yml
@@ -1,5 +1,6 @@
 id: d7_node_revision
 label: Node revisions
+audit: true
 migration_tags:
   - Drupal 7
 deriver: Drupal\node\Plugin\migrate\D7NodeDeriver
diff --git a/core/modules/taxonomy/migrations/d6_taxonomy_term.yml b/core/modules/taxonomy/migrations/d6_taxonomy_term.yml
index e3c3e3d342..9eafee544c 100644
--- a/core/modules/taxonomy/migrations/d6_taxonomy_term.yml
+++ b/core/modules/taxonomy/migrations/d6_taxonomy_term.yml
@@ -1,5 +1,6 @@
 id: d6_taxonomy_term
 label: Taxonomy terms
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/taxonomy/migrations/d6_term_node_revision.yml b/core/modules/taxonomy/migrations/d6_term_node_revision.yml
index 91c8362e63..c3ebe3059f 100644
--- a/core/modules/taxonomy/migrations/d6_term_node_revision.yml
+++ b/core/modules/taxonomy/migrations/d6_term_node_revision.yml
@@ -1,5 +1,6 @@
 id: d6_term_node_revision
 label: Term/node relationship revisions
+audit: true
 migration_tags:
   - Drupal 6
 deriver: Drupal\taxonomy\Plugin\migrate\D6TermNodeDeriver
diff --git a/core/modules/taxonomy/migrations/d7_taxonomy_term.yml b/core/modules/taxonomy/migrations/d7_taxonomy_term.yml
index 46f9f20427..6033d4f875 100644
--- a/core/modules/taxonomy/migrations/d7_taxonomy_term.yml
+++ b/core/modules/taxonomy/migrations/d7_taxonomy_term.yml
@@ -1,5 +1,6 @@
 id: d7_taxonomy_term
 label: Taxonomy terms
+audit: true
 migration_tags:
   - Drupal 7
 deriver: Drupal\taxonomy\Plugin\migrate\D7TaxonomyTermDeriver
diff --git a/core/modules/user/migrations/d6_user.yml b/core/modules/user/migrations/d6_user.yml
index d58607b150..35d31da875 100644
--- a/core/modules/user/migrations/d6_user.yml
+++ b/core/modules/user/migrations/d6_user.yml
@@ -1,5 +1,6 @@
 id: d6_user
 label: User accounts
+audit: true
 migration_tags:
   - Drupal 6
 source:
diff --git a/core/modules/user/migrations/d7_user.yml b/core/modules/user/migrations/d7_user.yml
index 54c8805875..ee70db782e 100644
--- a/core/modules/user/migrations/d7_user.yml
+++ b/core/modules/user/migrations/d7_user.yml
@@ -1,5 +1,6 @@
 id: d7_user
 label: User accounts
+audit: true
 migration_tags:
   - Drupal 7
 class: Drupal\user\Plugin\migrate\User
diff --git a/core/modules/user/src/Plugin/migrate/destination/EntityUser.php b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
index 7c2c81af96..07d01656fd 100644
--- a/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
+++ b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
@@ -127,4 +127,18 @@ protected function processStubRow(Row $row) {
     }
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getHighestId() {
+    $highest_id = parent::getHighestId();
+
+    // Every Drupal site must have a user with UID of 1 and it's normal for
+    // migrations to overwrite this user.
+    if ($highest_id === 1) {
+      return 0;
+    }
+    return $highest_id;
+  }
+
 }