diff --git a/core/modules/aggregator/migration_templates/d6_aggregator_feed.yml b/core/modules/aggregator/migration_templates/d6_aggregator_feed.yml
index cad1553..105d09b 100644
--- a/core/modules/aggregator/migration_templates/d6_aggregator_feed.yml
+++ b/core/modules/aggregator/migration_templates/d6_aggregator_feed.yml
@@ -2,6 +2,7 @@ id: d6_aggregator_feed
 label: Aggregator feeds
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: aggregator_feed
 process:
diff --git a/core/modules/aggregator/migration_templates/d6_aggregator_item.yml b/core/modules/aggregator/migration_templates/d6_aggregator_item.yml
index e14dbd6..022a780 100644
--- a/core/modules/aggregator/migration_templates/d6_aggregator_item.yml
+++ b/core/modules/aggregator/migration_templates/d6_aggregator_item.yml
@@ -2,6 +2,7 @@ id: d6_aggregator_item
 label: Aggregator items
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: aggregator_item
 process:
diff --git a/core/modules/aggregator/migration_templates/d7_aggregator_feed.yml b/core/modules/aggregator/migration_templates/d7_aggregator_feed.yml
index 5dbeb25..9a038b7 100644
--- a/core/modules/aggregator/migration_templates/d7_aggregator_feed.yml
+++ b/core/modules/aggregator/migration_templates/d7_aggregator_feed.yml
@@ -2,6 +2,7 @@ id: d7_aggregator_feed
 label: Aggregator feeds
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: aggregator_feed
 process:
diff --git a/core/modules/aggregator/migration_templates/d7_aggregator_item.yml b/core/modules/aggregator/migration_templates/d7_aggregator_item.yml
index 054ba43..ac76a8c 100644
--- a/core/modules/aggregator/migration_templates/d7_aggregator_item.yml
+++ b/core/modules/aggregator/migration_templates/d7_aggregator_item.yml
@@ -2,6 +2,7 @@ id: d7_aggregator_item
 label: Aggregator items
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: aggregator_item
 process:
diff --git a/core/modules/ban/migration_templates/d7_blocked_ips.yml b/core/modules/ban/migration_templates/d7_blocked_ips.yml
index 0122dc6..9197cf2 100644
--- a/core/modules/ban/migration_templates/d7_blocked_ips.yml
+++ b/core/modules/ban/migration_templates/d7_blocked_ips.yml
@@ -2,6 +2,7 @@ id: d7_blocked_ips
 label: Blocked IPs
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_blocked_ips
 process:
diff --git a/core/modules/block_content/migration_templates/d6_custom_block.yml b/core/modules/block_content/migration_templates/d6_custom_block.yml
index 55fbcb5..d2826c7 100644
--- a/core/modules/block_content/migration_templates/d6_custom_block.yml
+++ b/core/modules/block_content/migration_templates/d6_custom_block.yml
@@ -2,6 +2,7 @@ id: d6_custom_block
 label: Custom blocks
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_box
 process:
diff --git a/core/modules/block_content/migration_templates/d7_custom_block.yml b/core/modules/block_content/migration_templates/d7_custom_block.yml
index ca06cf0..1089ca3 100644
--- a/core/modules/block_content/migration_templates/d7_custom_block.yml
+++ b/core/modules/block_content/migration_templates/d7_custom_block.yml
@@ -2,6 +2,7 @@ id: d7_custom_block
 label: Custom blocks
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_block_custom
 process:
diff --git a/core/modules/book/migration_templates/d6_book.yml b/core/modules/book/migration_templates/d6_book.yml
index 94d7a8f..f075c39 100644
--- a/core/modules/book/migration_templates/d6_book.yml
+++ b/core/modules/book/migration_templates/d6_book.yml
@@ -2,6 +2,7 @@ id: d6_book
 label: Books
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_book
 process:
diff --git a/core/modules/comment/migration_templates/d6_comment.yml b/core/modules/comment/migration_templates/d6_comment.yml
index 06820d4..4add74b 100644
--- a/core/modules/comment/migration_templates/d6_comment.yml
+++ b/core/modules/comment/migration_templates/d6_comment.yml
@@ -2,6 +2,7 @@ id: d6_comment
 label: Comments
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_comment
   constants:
diff --git a/core/modules/comment/migration_templates/d7_comment.yml b/core/modules/comment/migration_templates/d7_comment.yml
index 94a2884..c020ab2 100644
--- a/core/modules/comment/migration_templates/d7_comment.yml
+++ b/core/modules/comment/migration_templates/d7_comment.yml
@@ -2,6 +2,7 @@ id: d7_comment
 label: Comments
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_comment
   constants:
diff --git a/core/modules/file/migration_templates/d6_file.yml b/core/modules/file/migration_templates/d6_file.yml
index 8371d45..8589978 100644
--- a/core/modules/file/migration_templates/d6_file.yml
+++ b/core/modules/file/migration_templates/d6_file.yml
@@ -4,6 +4,7 @@ id: d6_file
 label: Files
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_file
   constants:
diff --git a/core/modules/file/migration_templates/d6_upload.yml b/core/modules/file/migration_templates/d6_upload.yml
index a497099..0942e10 100644
--- a/core/modules/file/migration_templates/d6_upload.yml
+++ b/core/modules/file/migration_templates/d6_upload.yml
@@ -2,6 +2,7 @@ id: d6_upload
 label: File uploads
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_upload
 process:
diff --git a/core/modules/file/migration_templates/d7_file.yml b/core/modules/file/migration_templates/d7_file.yml
index 3fee046..8bc2147 100644
--- a/core/modules/file/migration_templates/d7_file.yml
+++ b/core/modules/file/migration_templates/d7_file.yml
@@ -4,6 +4,7 @@ id: d7_file
 label: Files
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_file
   scheme: public
diff --git a/core/modules/file/migration_templates/d7_file_private.yml b/core/modules/file/migration_templates/d7_file_private.yml
index 9c6b8e2..d2e7c89 100644
--- a/core/modules/file/migration_templates/d7_file_private.yml
+++ b/core/modules/file/migration_templates/d7_file_private.yml
@@ -2,6 +2,7 @@ id: d7_file_private
 label: Files
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_file
   scheme: private
diff --git a/core/modules/menu_link_content/migration_templates/d6_menu_links.yml b/core/modules/menu_link_content/migration_templates/d6_menu_links.yml
index 2c8ad4a..2a79d35 100644
--- a/core/modules/menu_link_content/migration_templates/d6_menu_links.yml
+++ b/core/modules/menu_link_content/migration_templates/d6_menu_links.yml
@@ -2,6 +2,7 @@ id: d6_menu_links
 label: Menu links
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: menu_link
 process:
diff --git a/core/modules/menu_link_content/migration_templates/d7_menu_links.yml b/core/modules/menu_link_content/migration_templates/d7_menu_links.yml
index 200a792..854a056 100644
--- a/core/modules/menu_link_content/migration_templates/d7_menu_links.yml
+++ b/core/modules/menu_link_content/migration_templates/d7_menu_links.yml
@@ -2,6 +2,7 @@ id: d7_menu_links
 label: Menu links
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: menu_link
   constants:
diff --git a/core/modules/migrate/migrate.services.yml b/core/modules/migrate/migrate.services.yml
index 1a4f64d..1b95191 100644
--- a/core/modules/migrate/migrate.services.yml
+++ b/core/modules/migrate/migrate.services.yml
@@ -30,3 +30,5 @@ services:
   plugin.manager.migration:
     class: Drupal\migrate\Plugin\MigrationPluginManager
     arguments: ['@module_handler', '@cache.discovery_migration', '@language_manager']
+  migrate.id_auditor:
+    class: Drupal\migrate\MigrateIdAuditor
diff --git a/core/modules/migrate/src/MigrateIdAuditor.php b/core/modules/migrate/src/MigrateIdAuditor.php
new file mode 100644
index 0000000..19f92cb
--- /dev/null
+++ b/core/modules/migrate/src/MigrateIdAuditor.php
@@ -0,0 +1,35 @@
+<?php
+
+namespace Drupal\migrate;
+
+use Drupal\migrate\Plugin\MigrateIdAuditInterface;
+
+/**
+ * Audits migrations for potential ID conflicts.
+ */
+class MigrateIdAuditor {
+
+  /**
+   * Audit a set of migrations for ID conflicts.
+   *
+   * @param \Drupal\migrate\Plugin\MigrationInterface[] $migrations
+   *   The migrations to audit.
+   *
+   * @return string[]
+   *   The entity type IDs of migrated content that may have problematic
+   *   IDs. If no problems are found, an empty array will be returned.
+   */
+  public function auditIds(array $migrations) {
+    $ret = [];
+    foreach ($migrations as $migration) {
+      $destination = $migration->getDestinationPlugin();
+      if ($destination instanceof MigrateIdAuditInterface) {
+        if ($destination->unsafeIdsExist($migration->getIdMap())) {
+          $ret[$destination->entityTypeId()] = TRUE;
+        }
+      }
+    }
+    return array_keys($ret);
+  }
+
+}
diff --git a/core/modules/migrate/src/Plugin/MigrateIdAuditInterface.php b/core/modules/migrate/src/Plugin/MigrateIdAuditInterface.php
new file mode 100644
index 0000000..f582210
--- /dev/null
+++ b/core/modules/migrate/src/Plugin/MigrateIdAuditInterface.php
@@ -0,0 +1,39 @@
+<?php
+
+namespace Drupal\migrate\Plugin;
+
+/**
+ * Should be implemented by destinations that are able to audit whether
+ * they may encounter unsafe ID mappings.
+ */
+interface MigrateIdAuditInterface {
+
+  /**
+   * Check whether unsafe IDs exist that should inhibit migration.
+   *
+   * @param MigrateIdMapInterface $id_map
+   *   The ID map for this migration.
+   *
+   * @return bool
+   *   Whether unsafe IDs exist.
+   */
+  public function unsafeIdsExist(MigrateIdMapInterface $id_map);
+
+  /**
+   * Get the subfield that contains the highest ID migrated by this
+   * destination.
+   *
+   * @return string
+   *   The field name.
+   */
+  public function getHighestIdField();
+
+  /**
+   * Get the type ID of the entities this destination creates.
+   *
+   * @return string
+   *   The type ID, eg: 'node'.
+   */
+  public function entityTypeId();
+
+}
diff --git a/core/modules/migrate/src/Plugin/MigrateMaxIdInterface.php b/core/modules/migrate/src/Plugin/MigrateMaxIdInterface.php
new file mode 100644
index 0000000..a8138a8
--- /dev/null
+++ b/core/modules/migrate/src/Plugin/MigrateMaxIdInterface.php
@@ -0,0 +1,19 @@
+<?php
+
+namespace Drupal\migrate\Plugin;
+
+interface MigrateMaxIdInterface {
+
+  /**
+   * Gets the highest destination ID value from an ID map.
+   *
+   * @param string $field
+   *   The destination field for which to get the highest ID.
+   *
+   * @return int
+   *   The highest ID value found. If no IDs at all are found, or if the
+   *   concept of a highest ID is not meaningful, zero should be returned.
+   */
+  public function getMaxId($field);
+
+}
diff --git a/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
index d4f9bd7..b043b28 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityContentBase.php
@@ -9,6 +9,8 @@
 use Drupal\Core\Field\FieldTypePluginManagerInterface;
 use Drupal\Core\TypedData\TranslatableInterface;
 use Drupal\Core\TypedData\TypedDataInterface;
+use Drupal\migrate\Plugin\MigrateMaxIdInterface;
+use Drupal\migrate\Plugin\MigrateIdAuditInterface;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\MigrateException;
 use Drupal\migrate\Plugin\MigrateIdMapInterface;
@@ -17,8 +19,14 @@
 
 /**
  * The destination class for all content entities lacking a specific class.
+ *
+ * Configuration options:
+ *
+ *   - audit_ids: If set to TRUE, indicates that IDs are audited by this
+ *     migration. This means that the source IDs will be audited to make sure
+ *     the latest source id will not overwrite anything in the destination.
  */
-class EntityContentBase extends Entity {
+class EntityContentBase extends Entity implements MigrateIdAuditInterface {
 
   /**
    * Entity manager.
@@ -289,4 +297,54 @@ protected function getDefinitionFromEntity($key) {
     ] + $field_definition->getSettings();
   }
 
+  /**
+   * Get the highest ID that exists for this destination.
+   *
+   * This is not the highest ID that has been migrated, but the highest ID
+   * that exists in the destination, eg: highest node ID on the site.
+   *
+   * @return int
+   *   The highest ID value found. If no IDs at all are found, or if the
+   *   concept of a highest ID is not meaningful, zero should be returned.
+   */
+  protected function highestDestinationId() {
+    $query = $this->storage->getQuery()
+      ->sort($this->getHighestIdField(), 'DESC')
+      ->range(0, 1);
+    $found = $query->execute();
+    return (int) reset($found);
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function getHighestIdField() {
+    return $this->getKey('id');
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function unsafeIdsExist(MigrateIdMapInterface $id_map) {
+    // If IDs are are audited, see if there are any conflicts.
+    if (!empty($this->migration->getPluginDefinition()['audit_ids'])) {
+      if (!($id_map instanceof MigrateMaxIdInterface)) {
+        // We don't know how to audit IDs without a cooperating ID map.
+        return FALSE;
+      }
+      $highestMigrated = $id_map->getMaxId($this->getHighestIdField());
+      if ($this->highestDestinationId() > $highestMigrated) {
+        // There's a new ID that we might conflict with!
+        return TRUE;
+      }
+    }
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function entityTypeId() {
+    return $this->storage->getEntityTypeId();
+  }
+
 }
diff --git a/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php b/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
index b0db476..b646059 100644
--- a/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
+++ b/core/modules/migrate/src/Plugin/migrate/destination/EntityRevision.php
@@ -78,4 +78,11 @@ public function getIds() {
     throw new MigrateException('This entity type does not support revisions.');
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getHighestIdField() {
+    return $this->getKey('revision');
+  }
+
 }
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 27b7569..2b6e30c 100644
--- a/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
+++ b/core/modules/migrate/src/Plugin/migrate/id_map/Sql.php
@@ -6,6 +6,7 @@
 use Drupal\Core\Field\BaseFieldDefinition;
 use Drupal\Core\Plugin\ContainerFactoryPluginInterface;
 use Drupal\Core\Plugin\PluginBase;
+use Drupal\migrate\Plugin\MigrateMaxIdInterface;
 use Drupal\migrate\Plugin\MigrationInterface;
 use Drupal\migrate\Event\MigrateIdMapMessageEvent;
 use Drupal\migrate\MigrateException;
@@ -26,7 +27,7 @@
  *
  * @PluginID("sql")
  */
-class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryPluginInterface {
+class Sql extends PluginBase implements MigrateIdMapInterface, ContainerFactoryPluginInterface, MigrateMaxIdInterface {
 
   /**
    * Column name of hashed source id values.
@@ -923,4 +924,22 @@ public function valid() {
     return $this->currentRow !== FALSE;
   }
 
+  /**
+   * {@inheritdoc}
+   */
+  public function getMaxId($field) {
+    if (!$this->getDatabase()->schema()->tableExists($this->mapTableName())) {
+      return 0;
+    }
+
+    $sqlField = $this->destinationIdFields()[$field];
+
+    $query = $this->getDatabase()->select($this->mapTableName(), 'map')
+      ->fields('map', [$sqlField])
+      ->orderBy($sqlField, 'DESC')
+      ->range(0, 1);
+    $found = $query->execute()->fetchField();
+    return (int) $found;
+  }
+
 }
diff --git a/core/modules/migrate/tests/modules/migrate_high_water_test/migration_templates/migrate.migration.high_water_test.yml b/core/modules/migrate/tests/modules/migrate_high_water_test/migration_templates/migrate.migration.high_water_test.yml
index 6a86577..73f32a8 100644
--- a/core/modules/migrate/tests/modules/migrate_high_water_test/migration_templates/migrate.migration.high_water_test.yml
+++ b/core/modules/migrate/tests/modules/migrate_high_water_test/migration_templates/migrate.migration.high_water_test.yml
@@ -1,5 +1,6 @@
 id: high_water_test
 label: High water test.
+audit_ids: true
 source:
   plugin: high_water_test
   high_water_property:
diff --git a/core/modules/migrate_drupal/tests/modules/migrate_overwrite_test/migration_templates/users.yml b/core/modules/migrate_drupal/tests/modules/migrate_overwrite_test/migration_templates/users.yml
index e23d90d..0bddcf4 100644
--- a/core/modules/migrate_drupal/tests/modules/migrate_overwrite_test/migration_templates/users.yml
+++ b/core/modules/migrate_drupal/tests/modules/migrate_overwrite_test/migration_templates/users.yml
@@ -3,6 +3,7 @@ label: User migration
 migration_tags:
   - Drupal 6
   - Drupal 7
+audit_ids: true
 source:
   plugin: d6_user
 process:
diff --git a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
index c87247e..53a2a46 100644
--- a/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
+++ b/core/modules/migrate_drupal_ui/src/Form/MigrateUpgradeForm.php
@@ -3,11 +3,13 @@
 namespace Drupal\migrate_drupal_ui\Form;
 
 use Drupal\Core\Datetime\DateFormatterInterface;
+use Drupal\Core\Entity\EntityTypeManagerInterface;
 use Drupal\Core\Form\ConfirmFormBase;
 use Drupal\Core\Form\FormStateInterface;
 use Drupal\Core\Render\RendererInterface;
 use Drupal\Core\State\StateInterface;
 use Drupal\Core\Url;
+use Drupal\migrate\MigrateIdAuditor;
 use Drupal\migrate\Plugin\MigrationPluginManagerInterface;
 use Drupal\migrate_drupal_ui\Batch\MigrateUpgradeImportBatch;
 use Drupal\migrate_drupal\MigrationConfigurationTrait;
@@ -713,6 +715,20 @@ class MigrateUpgradeForm extends ConfirmFormBase {
   protected $pluginManager;
 
   /**
+   * The entity type manager.
+   *
+   * @var \Drupal\Core\Entity\EntityTypeManagerInterface
+   */
+  protected $entityTypeManager;
+
+  /**
+   * The ID conflict auditor.
+   *
+   * @var \Drupal\migrate\MigrateIdAuditor $idAuditor
+   */
+  protected $idAuditor;
+
+  /**
    * Constructs the MigrateUpgradeForm.
    *
    * @param \Drupal\Core\State\StateInterface $state
@@ -723,12 +739,18 @@ class MigrateUpgradeForm extends ConfirmFormBase {
    *   The renderer service.
    * @param \Drupal\migrate\Plugin\MigrationPluginManagerInterface $plugin_manager
    *   The migration plugin manager.
+   * @param \Drupal\Core\Entity\EntityTypeManagerInterface $entity_type_manager
+   *   The entity type manager.
+   * @param \Drupal\migrate\MigrateIdAuditor $id_auditor
+   *   The ID conflict auditor.
    */
-  public function __construct(StateInterface $state, DateFormatterInterface $date_formatter, RendererInterface $renderer, MigrationPluginManagerInterface $plugin_manager) {
+  public function __construct(StateInterface $state, DateFormatterInterface $date_formatter, RendererInterface $renderer, MigrationPluginManagerInterface $plugin_manager, EntityTypeManagerInterface $entity_type_manager, MigrateIdAuditor $id_auditor) {
     $this->state = $state;
     $this->dateFormatter = $date_formatter;
     $this->renderer = $renderer;
     $this->pluginManager = $plugin_manager;
+    $this->entityTypeManager = $entity_type_manager;
+    $this->idAuditor = $id_auditor;
   }
 
   /**
@@ -739,7 +761,9 @@ public static function create(ContainerInterface $container) {
       $container->get('state'),
       $container->get('date.formatter'),
       $container->get('renderer'),
-      $container->get('plugin.manager.migration')
+      $container->get('plugin.manager.migration'),
+      $container->get('entity_type.manager'),
+      $container->get('migrate.id_auditor')
     );
   }
 
@@ -762,6 +786,9 @@ 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);
 
@@ -1082,6 +1109,64 @@ 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);
+    $type_ids = $this->idAuditor->auditIds($migrations);
+    if (empty($type_ids)) {
+      $form_state->set('step', 'confirm');
+      return $this->buildForm($form, $form_state);
+    }
+
+    $form = parent::buildForm($form, $form_state);
+    $form['actions']['submit']['#submit'] = ['::submitConfirmIdConflictForm'];
+    $form['actions']['submit']['#value'] = $this->t('I acknowledge I may lose data, continue anyway.');
+
+    $form['warning'] = [
+      '#type' => 'markup',
+      '#markup' => '<h3>' . $this->t('Entities may be overwritten') . '</h3>' .
+        '<p>' . $this->t('Upgrades work on brand new sites, or sites with previously upgraded data. 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>',
+    ];
+
+    $items = [];
+    sort($type_ids);
+    foreach ($type_ids as $typeId) {
+      $items[] = $this->entityTypeManager->getDefinition($typeId)->getLabel();
+    }
+    $form['type_list'] = [
+      '#title' => $this->t('These entities are of the following types:'),
+      '#theme' => 'item_list',
+      '#items' => $items,
+    ];
+
+    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 9741a0b..b88fbb1 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/MigrateUpgradeTestBase.php
@@ -37,7 +37,6 @@
     'telephone',
     'aggregator',
     'book',
-    'forum',
     'statistics',
   ];
 
@@ -111,6 +110,29 @@ protected function tearDown() {
    * Executes all steps of migrations upgrade.
    */
   public function testMigrateUpgrade() {
+    $driver = $this->sourceDatabase->getConnectionOptions()['driver'];
+    $edits = $this->getUiEditsArray();
+
+    // Ensure submitting the form with invalid database credentials gives us a
+    // nice warning.
+    $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
+    $this->assertText('Resolve the issue below to continue the upgrade.');
+
+    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->assertResponse(200);
+    $this->assertText('Are you sure?');
+    $this->drupalPostForm(NULL, [], t('Perform upgrade'));
+    $this->assertText(t('Congratulations, you upgraded Drupal!'));
+
+    $this->runPostMigrationTests();
+  }
+
+  /**
+   * Build a re-usable form $edits array for the UI.
+   *
+   * @return array $edits
+   */
+  protected function getUiEditsArray() {
     $connection_options = $this->sourceDatabase->getConnectionOptions();
     $this->drupalGet('/upgrade');
     $this->assertText('Upgrade a site by importing it into a clean and empty new install of Drupal 8. You will lose any existing configuration once you import your site into it. See the online documentation for Drupal site upgrades for more detailed information.');
@@ -137,29 +159,23 @@ public function testMigrateUpgrade() {
     if (count($drivers) !== 1) {
       $edit['driver'] = $driver;
     }
-    $edits = $this->translatePostValues($edit);
-
-    // Ensure submitting the form with invalid database credentials gives us a
-    // nice warning.
-    $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
-    $this->assertText('Resolve the issue below to continue the upgrade.');
-
-    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
-    $this->assertResponse(200);
-    $this->assertText('Are you sure?');
-    $this->drupalPostForm(NULL, [], t('Perform upgrade'));
-    $this->assertText(t('Congratulations, you upgraded Drupal!'));
+    return $this->translatePostValues($edit);
+  }
 
+  /**
+   * Runs all post migration tests.
+   */
+  protected function runPostMigrationTests() {
+    $version = $this->getLegacyDrupalVersion($this->sourceDatabase);
     // Have to reset all the statics after migration to ensure entities are
     // loadable.
     $this->resetAll();
 
     $expected_counts = $this->getEntityCounts();
-    foreach (array_keys(\Drupal::entityTypeManager()
-      ->getDefinitions()) as $entity_type) {
+    foreach (array_keys(\Drupal::entityTypeManager()->getDefinitions()) as $entity_type) {
       $real_count = \Drupal::entityQuery($entity_type)->count()->execute();
       $expected_count = isset($expected_counts[$entity_type]) ? $expected_counts[$entity_type] : 0;
-      $this->assertEqual($expected_count, $real_count, "Found $real_count $entity_type entities, expected $expected_count.");
+      $this->assertSame($expected_count, $real_count, "Found $real_count $entity_type entities, expected $expected_count.");
     }
 
     $plugin_manager = \Drupal::service('plugin.manager.migration');
@@ -182,12 +198,10 @@ public function testMigrateUpgrade() {
           $this->fail($message);
         }
         else {
-          $this->pass($message);
+          $this->assertTrue(TRUE, $message);
         }
       }
     }
-    \Drupal::service('module_installer')->install(['forum']);
-    \Drupal::service('module_installer')->install(['book']);
   }
 
   /**
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
index 1c4b883..24a0d2c 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgrade6Test.php
@@ -17,6 +17,19 @@ class MigrateUpgrade6Test extends MigrateUpgradeTestBase {
   /**
    * {@inheritdoc}
    */
+  public static $modules = [
+    'language',
+    'content_translation',
+    'migrate_drupal_ui',
+    'telephone',
+    'aggregator',
+    'book',
+    'statistics',
+  ];
+
+  /**
+   * {@inheritdoc}
+   */
   protected function setUp() {
     parent::setUp();
     $this->loadFixture(drupal_get_path('module', 'migrate_drupal') . '/tests/fixtures/drupal6.php');
@@ -40,7 +53,7 @@ protected function getEntityCounts() {
       'block_content' => 2,
       'block_content_type' => 1,
       'comment' => 3,
-      'comment_type' => 3,
+      'comment_type' => 2,
       'contact_form' => 5,
       'configurable_language' => 5,
       'editor' => 2,
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgradeConflicts6Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgradeConflicts6Test.php
new file mode 100644
index 0000000..642bb9e
--- /dev/null
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d6/MigrateUpgradeConflicts6Test.php
@@ -0,0 +1,60 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal_ui\Functional\d6;
+
+/**
+ * Tests Drupal 6 upgrade using the migrate UI and has ID conflicts.
+ *
+ * The test method is provided by the MigrateUpgradeTestBase class.
+ *
+ * @group migrate_drupal_ui
+ */
+class MigrateUpgradeConflicts6Test extends MigrateUpgrade6Test {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'language',
+    'content_translation',
+    'migrate_drupal_ui',
+    'telephone',
+    'aggregator',
+    'book',
+    // Forum module adds content, resulting in content id conflicts.
+    'forum',
+    'statistics',
+  ];
+
+  /**
+   * Executes all steps of migrations upgrade.
+   */
+  public function testMigrateUpgrade() {
+    $driver = $this->sourceDatabase->getConnectionOptions()['driver'];
+    $edits = $this->getUiEditsArray();
+
+    // Ensure submitting the form with invalid database credentials gives us a
+    // nice warning.
+    $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
+    $this->assertText('Resolve the issue below to continue the upgrade.');
+
+    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->assertResponse(200);
+    $this->assertText('Are you sure?');
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
+    $this->drupalPostForm(NULL, [], t('Perform upgrade'));
+    $this->assertText(t('Congratulations, you upgraded Drupal!'));
+
+    $this->runPostMigrationTests();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEntityCounts() {
+    $counts = parent::getEntityCounts();
+    $counts['comment_type'] = 3;
+    return $counts;
+  }
+
+}
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
index 248213a..6cadfe5 100644
--- a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgrade7Test.php
@@ -45,7 +45,7 @@ protected function getEntityCounts() {
       'block_content' => 1,
       'block_content_type' => 1,
       'comment' => 1,
-      'comment_type' => 8,
+      'comment_type' => 7,
       // Module 'language' comes with 'en', 'und', 'zxx'. Migration adds 'is'.
       'configurable_language' => 4,
       'contact_form' => 3,
diff --git a/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgradeConflicts7Test.php b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgradeConflicts7Test.php
new file mode 100644
index 0000000..5bba621
--- /dev/null
+++ b/core/modules/migrate_drupal_ui/tests/src/Functional/d7/MigrateUpgradeConflicts7Test.php
@@ -0,0 +1,62 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal_ui\Functional\d7;
+
+/**
+ * Tests Drupal 7 upgrade using the migrate UI and has ID conflicts.
+ *
+ * The test method is provided by the MigrateUpgradeTestBase class.
+ *
+ * @group migrate_drupal_ui
+ */
+class MigrateUpgradeConflicts7Test extends MigrateUpgrade7Test {
+
+  /**
+   * {@inheritdoc}
+   */
+  public static $modules = [
+    'file',
+    'language',
+    'content_translation',
+    'migrate_drupal_ui',
+    'telephone',
+    'aggregator',
+    'book',
+    // Forum module adds content, resulting in content id conflicts.
+    'forum',
+    'statistics',
+  ];
+
+  /**
+   * Executes all steps of migrations upgrade.
+   */
+  public function testMigrateUpgrade() {
+    $driver = $this->sourceDatabase->getConnectionOptions()['driver'];
+    $edits = $this->getUiEditsArray();
+
+    // Ensure submitting the form with invalid database credentials gives us a
+    // nice warning.
+    $this->drupalPostForm(NULL, [$driver . '[database]' => 'wrong'] + $edits, t('Review upgrade'));
+    $this->assertText('Resolve the issue below to continue the upgrade.');
+
+    $this->drupalPostForm(NULL, $edits, t('Review upgrade'));
+    $this->assertResponse(200);
+    $this->assertText('Are you sure?');
+    $this->drupalPostForm(NULL, [], t('I acknowledge I may lose data, continue anyway.'));
+    $this->drupalPostForm(NULL, [], t('Perform upgrade'));
+    $this->assertText(t('Congratulations, you upgraded Drupal!'));
+
+    $this->runPostMigrationTests();
+  }
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEntityCounts() {
+    $counts = parent::getEntityCounts();
+    // Forum has a comment.
+    $counts['comment_type'] = 8;
+    return $counts;
+  }
+
+}
diff --git a/core/modules/node/migration_templates/d6_node.yml b/core/modules/node/migration_templates/d6_node.yml
index 56d0459..1bcae10 100644
--- a/core/modules/node/migration_templates/d6_node.yml
+++ b/core/modules/node/migration_templates/d6_node.yml
@@ -3,6 +3,7 @@ label: Nodes
 migration_tags:
   - Drupal 6
 deriver: Drupal\node\Plugin\migrate\D6NodeDeriver
+audit_ids: true
 source:
   plugin: d6_node
 process:
diff --git a/core/modules/node/migration_templates/d6_node_revision.yml b/core/modules/node/migration_templates/d6_node_revision.yml
index f4ff301..ec44d58 100644
--- a/core/modules/node/migration_templates/d6_node_revision.yml
+++ b/core/modules/node/migration_templates/d6_node_revision.yml
@@ -3,6 +3,7 @@ label: Node revisions
 migration_tags:
   - Drupal 6
 deriver: Drupal\node\Plugin\migrate\D6NodeDeriver
+audit_ids: true
 source:
   plugin: d6_node_revision
 process:
diff --git a/core/modules/node/migration_templates/d6_node_translation.yml b/core/modules/node/migration_templates/d6_node_translation.yml
index 3eb06e8..0f6579e 100644
--- a/core/modules/node/migration_templates/d6_node_translation.yml
+++ b/core/modules/node/migration_templates/d6_node_translation.yml
@@ -3,6 +3,7 @@ label: Node translations
 migration_tags:
   - Drupal 6
 deriver: Drupal\node\Plugin\migrate\D6NodeDeriver
+audit_ids: true
 source:
   plugin: d6_node
   translations: true
diff --git a/core/modules/node/migration_templates/d7_node.yml b/core/modules/node/migration_templates/d7_node.yml
index 5de3055..824662f 100644
--- a/core/modules/node/migration_templates/d7_node.yml
+++ b/core/modules/node/migration_templates/d7_node.yml
@@ -3,6 +3,7 @@ label: Nodes
 migration_tags:
   - Drupal 7
 deriver: Drupal\node\Plugin\migrate\D7NodeDeriver
+audit_ids: true
 source:
   plugin: d7_node
 process:
diff --git a/core/modules/node/migration_templates/d7_node_revision.yml b/core/modules/node/migration_templates/d7_node_revision.yml
index c6081ef..420fe58 100644
--- a/core/modules/node/migration_templates/d7_node_revision.yml
+++ b/core/modules/node/migration_templates/d7_node_revision.yml
@@ -3,6 +3,7 @@ label: Node revisions
 migration_tags:
   - Drupal 7
 deriver: Drupal\node\Plugin\migrate\D7NodeDeriver
+audit_ids: true
 source:
   plugin: d7_node_revision
 process:
diff --git a/core/modules/node/migration_templates/d7_node_translation.yml b/core/modules/node/migration_templates/d7_node_translation.yml
index 11dfbb7..5a073d7 100644
--- a/core/modules/node/migration_templates/d7_node_translation.yml
+++ b/core/modules/node/migration_templates/d7_node_translation.yml
@@ -4,6 +4,7 @@ migration_tags:
   - Drupal 7
   - translation
 deriver: Drupal\node\Plugin\migrate\D7NodeDeriver
+audit_ids: true
 source:
   plugin: d7_node
   translations: true
diff --git a/core/modules/path/migration_templates/d6_url_alias.yml b/core/modules/path/migration_templates/d6_url_alias.yml
index fcf2036..f9c604c 100644
--- a/core/modules/path/migration_templates/d6_url_alias.yml
+++ b/core/modules/path/migration_templates/d6_url_alias.yml
@@ -2,6 +2,7 @@ id: d6_url_alias
 label: URL aliases
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_url_alias
   constants:
diff --git a/core/modules/path/migration_templates/d7_url_alias.yml b/core/modules/path/migration_templates/d7_url_alias.yml
index dffaf46..12a985c 100644
--- a/core/modules/path/migration_templates/d7_url_alias.yml
+++ b/core/modules/path/migration_templates/d7_url_alias.yml
@@ -2,6 +2,7 @@ id: d7_url_alias
 label: URL aliases
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_url_alias
   constants:
diff --git a/core/modules/shortcut/migration_templates/d7_shortcut.yml b/core/modules/shortcut/migration_templates/d7_shortcut.yml
index dac9354..a597780 100644
--- a/core/modules/shortcut/migration_templates/d7_shortcut.yml
+++ b/core/modules/shortcut/migration_templates/d7_shortcut.yml
@@ -2,6 +2,7 @@ id: d7_shortcut
 label: Shortcut links
 migration_tags:
   - Drupal 7
+audit_ids: false
 source:
   plugin: d7_shortcut
   constants:
diff --git a/core/modules/shortcut/migration_templates/d7_shortcut_set_users.yml b/core/modules/shortcut/migration_templates/d7_shortcut_set_users.yml
index d0eb219..9726732 100644
--- a/core/modules/shortcut/migration_templates/d7_shortcut_set_users.yml
+++ b/core/modules/shortcut/migration_templates/d7_shortcut_set_users.yml
@@ -2,6 +2,7 @@ id: d7_shortcut_set_users
 label: Shortcut set user mapping
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_shortcut_set_users
 process:
diff --git a/core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml b/core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml
index e3c3e3d..7ed88a7 100644
--- a/core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml
+++ b/core/modules/taxonomy/migration_templates/d6_taxonomy_term.yml
@@ -2,6 +2,7 @@ id: d6_taxonomy_term
 label: Taxonomy terms
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_taxonomy_term
 process:
diff --git a/core/modules/taxonomy/migration_templates/d6_taxonomy_term_translation.yml b/core/modules/taxonomy/migration_templates/d6_taxonomy_term_translation.yml
index 11af8cd..473ab23 100644
--- a/core/modules/taxonomy/migration_templates/d6_taxonomy_term_translation.yml
+++ b/core/modules/taxonomy/migration_templates/d6_taxonomy_term_translation.yml
@@ -2,6 +2,7 @@ id: d6_taxonomy_term_translation
 label: Taxonomy terms
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_taxonomy_term
   translations: true
diff --git a/core/modules/taxonomy/migration_templates/d6_taxonomy_vocabulary_translation.yml b/core/modules/taxonomy/migration_templates/d6_taxonomy_vocabulary_translation.yml
index dbb1793..2d143d0 100644
--- a/core/modules/taxonomy/migration_templates/d6_taxonomy_vocabulary_translation.yml
+++ b/core/modules/taxonomy/migration_templates/d6_taxonomy_vocabulary_translation.yml
@@ -2,6 +2,7 @@ id: d6_taxonomy_vocabulary_translation
 label: Taxonomy vocabularies
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_taxonomy_vocabulary_translation
 process:
diff --git a/core/modules/taxonomy/migration_templates/d6_term_node.yml b/core/modules/taxonomy/migration_templates/d6_term_node.yml
index 846334d..bda7d58 100644
--- a/core/modules/taxonomy/migration_templates/d6_term_node.yml
+++ b/core/modules/taxonomy/migration_templates/d6_term_node.yml
@@ -3,6 +3,7 @@ label: Term/node relationships
 migration_tags:
   - Drupal 6
 deriver: Drupal\taxonomy\Plugin\migrate\D6TermNodeDeriver
+audit_ids: true
 source:
   plugin: d6_term_node
 process:
diff --git a/core/modules/taxonomy/migration_templates/d6_term_node_revision.yml b/core/modules/taxonomy/migration_templates/d6_term_node_revision.yml
index 91c8362..24da88f 100644
--- a/core/modules/taxonomy/migration_templates/d6_term_node_revision.yml
+++ b/core/modules/taxonomy/migration_templates/d6_term_node_revision.yml
@@ -3,6 +3,7 @@ label: Term/node relationship revisions
 migration_tags:
   - Drupal 6
 deriver: Drupal\taxonomy\Plugin\migrate\D6TermNodeDeriver
+audit_ids: true
 source:
   plugin: d6_term_node_revision
 process:
diff --git a/core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml b/core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml
index 99004df..e518ead 100644
--- a/core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml
+++ b/core/modules/taxonomy/migration_templates/d7_taxonomy_term.yml
@@ -3,6 +3,7 @@ label: Taxonomy terms
 migration_tags:
   - Drupal 7
 deriver: Drupal\taxonomy\Plugin\migrate\D7TaxonomyTermDeriver
+audit_ids: true
 source:
   plugin: d7_taxonomy_term
 process:
diff --git a/core/modules/tracker/migration_templates/d7_tracker_node.yml b/core/modules/tracker/migration_templates/d7_tracker_node.yml
index 02645d7..2bf3b07 100644
--- a/core/modules/tracker/migration_templates/d7_tracker_node.yml
+++ b/core/modules/tracker/migration_templates/d7_tracker_node.yml
@@ -2,6 +2,7 @@ id: d7_tracker_node
 label: Tracker node
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_tracker_node
 process:
diff --git a/core/modules/tracker/migration_templates/d7_tracker_user.yml b/core/modules/tracker/migration_templates/d7_tracker_user.yml
index ae3c51d..776e1c7 100644
--- a/core/modules/tracker/migration_templates/d7_tracker_user.yml
+++ b/core/modules/tracker/migration_templates/d7_tracker_user.yml
@@ -2,6 +2,7 @@ id: d7_tracker_user
 label: Tracker user
 migration_tags:
   - Drupal 7
+audit_ids: true
 source:
   plugin: d7_tracker_user
 process:
diff --git a/core/modules/user/migration_templates/d6_profile_values.yml b/core/modules/user/migration_templates/d6_profile_values.yml
index 5530ca5..8b0d4b0 100644
--- a/core/modules/user/migration_templates/d6_profile_values.yml
+++ b/core/modules/user/migration_templates/d6_profile_values.yml
@@ -3,6 +3,7 @@ label: Profile values
 class: Drupal\user\Plugin\migrate\ProfileValues
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_profile_field_values
 process:
diff --git a/core/modules/user/migration_templates/d6_user.yml b/core/modules/user/migration_templates/d6_user.yml
index d58607b..034ce22 100644
--- a/core/modules/user/migration_templates/d6_user.yml
+++ b/core/modules/user/migration_templates/d6_user.yml
@@ -2,6 +2,7 @@ id: d6_user
 label: User accounts
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_user
 process:
diff --git a/core/modules/user/migration_templates/d6_user_contact_settings.yml b/core/modules/user/migration_templates/d6_user_contact_settings.yml
index 0d9a228..3fc3c3f 100644
--- a/core/modules/user/migration_templates/d6_user_contact_settings.yml
+++ b/core/modules/user/migration_templates/d6_user_contact_settings.yml
@@ -2,6 +2,7 @@ id: d6_user_contact_settings
 label: User contact settings
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_user
   constants:
diff --git a/core/modules/user/migration_templates/d6_user_picture_file.yml b/core/modules/user/migration_templates/d6_user_picture_file.yml
index 3518d7a..93a6e5e 100644
--- a/core/modules/user/migration_templates/d6_user_picture_file.yml
+++ b/core/modules/user/migration_templates/d6_user_picture_file.yml
@@ -2,6 +2,7 @@ id: d6_user_picture_file
 label: User pictures
 migration_tags:
   - Drupal 6
+audit_ids: true
 source:
   plugin: d6_user_picture_file
   constants:
diff --git a/core/modules/user/migration_templates/d7_user.yml b/core/modules/user/migration_templates/d7_user.yml
index ae52384..9babed3 100644
--- a/core/modules/user/migration_templates/d7_user.yml
+++ b/core/modules/user/migration_templates/d7_user.yml
@@ -3,6 +3,7 @@ label: User accounts
 migration_tags:
   - Drupal 7
 class: Drupal\user\Plugin\migrate\User
+audit_ids: true
 source:
   plugin: d7_user
 process:
diff --git a/core/modules/user/src/Plugin/migrate/destination/EntityUser.php b/core/modules/user/src/Plugin/migrate/destination/EntityUser.php
index 7c2c81a..f0206e5 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}
+   */
+  protected function highestDestinationId() {
+    $found = parent::highestDestinationId();
+
+    // Every Drupal site must have at least a single non-anonymous user, and
+    // it's normal for upgrade migrations to overwrite this user.
+    if ($found == 1) {
+      return 0;
+    }
+    return $found;
+  }
+
 }