diff --git a/core/modules/migrate/src/Plugin/MigrateDestinationAuditInterface.php b/core/modules/migrate/src/Plugin/MigrateDestinationAuditInterface.php
index 9592679..df92efb 100644
--- a/core/modules/migrate/src/Plugin/MigrateDestinationAuditInterface.php
+++ b/core/modules/migrate/src/Plugin/MigrateDestinationAuditInterface.php
@@ -11,6 +11,11 @@
   /**
    * Checks whether ID conflicts exist.
    *
+   * ID conflicts exist if new content has been created without the migration
+   * system knowing about it. To find out if there is ID conflicts, this method
+   * should compare the highest destination ID to the highest migrated ID. If
+   * the former is greater than the later, there is ID conflicts.
+   *
    * @param MigrateIdMapInterface $id_map
    *   The ID map for this migration.
    *
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php b/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php
index 854bb1a..00ffb8d 100644
--- a/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php
+++ b/core/modules/migrate_drupal/tests/src/Kernel/d6/MigrateDrupal6AuditIdsTest.php
@@ -2,8 +2,16 @@
 
 namespace Drupal\Tests\migrate_drupal\Kernel\d6;
 
+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\KernelTests\FileSystemModuleDiscoveryDataProviderTrait;
 use Drupal\node\Entity\Node;
+use Drupal\taxonomy\Entity\Term;
+use Drupal\user\Entity\User;
 
 /**
  * Tests the migration auditor for ID conflicts.
@@ -31,46 +39,15 @@ protected function setUp() {
     $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 all migrations with no ID conflicts.
-   */
-  public function testAllMigrationsWithNoConflicts() {
-    $plugin_manager = $this->container->get('plugin.manager.migration');
-
-    // Get all migration definitions.
-    $migrations = [];
-    foreach ($plugin_manager->getDefinitions() as $migration_id => $migration) {
-      $migrations[$migration_id] = $this->getMigration($migration_id);
-    }
-
-    // Audit the IDs of all migrations. There should be no conflicts since no
-    // content has been created.
-    $conflicts = $this->container->get('migrate.id_auditor')->auditIds($migrations);
-    $this->assertTrue(empty($conflicts));
-  }
-
-  /**
-   * Tests the node migration with ID conflicts.
-   */
-  public function testMigrationWithIdConflicts() {
-    // Create a node of type page.
-    Node::create(['type' => 'page', 'title' => 'foo'])->save();
-
-    // Audit the IDs of the d6_node migrations for the page & article node type.
-    // There should be a conflict since there's a new node that wasn't created
-    // from a previous migration.
-    $conflicts = $this->container->get('migrate.id_auditor')->auditIds([
-      $this->getMigration('d6_node:article'),
-      $this->getMigration('d6_node:page'),
-    ]);
-    $this->assertSame(['node'], array_keys($conflicts));
-  }
-
-  /**
    * Tests multiple migrations to the same destination with no ID conflicts.
    */
   public function testMultipleMigrationWithoutIdConflicts() {
@@ -99,4 +76,126 @@ public function testMultipleMigrationWithoutIdConflicts() {
     $this->assertTrue(empty($conflicts));
   }
 
+  /**
+   * Tests all migrations with no ID conflicts.
+   */
+  public function testAllMigrationsWithNoIdConflicts() {
+    $plugin_manager = $this->container->get('plugin.manager.migration');
+
+    // Get all migration definitions.
+    $migrations = [];
+    foreach ($plugin_manager->getDefinitions() as $migration_id => $migration) {
+      $migrations[$migration_id] = $this->getMigration($migration_id);
+    }
+
+    // Audit the IDs of all migrations. There should be no conflicts since no
+    // content has been created.
+    $conflicts = $this->container->get('migrate.id_auditor')->auditIds($migrations);
+    $this->assertTrue(empty($conflicts));
+  }
+
+  /**
+   * Tests all migrations with ID conflicts.
+   */
+  public function testAllMigrationsWithIdConflicts() {
+    $plugin_manager = $this->container->get('plugin.manager.migration');
+
+    // Get all migration definitions.
+    $migrations = [];
+    foreach ($plugin_manager->getDefinitions() as $migration_id => $migration) {
+      $migrations[$migration_id] = $this->getMigration($migration_id);
+    }
+
+    // Create content.
+    $this->createContent();
+
+    // Audit the IDs of all migrations. There should be no conflicts since no
+    // content has been created.
+    $conflicts = $this->container->get('migrate.id_auditor')->auditIds($migrations);
+    ksort($conflicts);
+
+    $expected = [
+      'aggregator_feed',
+      'aggregator_item',
+      'block_content',
+      'comment',
+      'file',
+      'menu_link_content',
+      'node',
+      'taxonomy_term',
+      'user',
+    ];
+
+    $this->assertSame($expected, array_keys($conflicts));
+  }
+
+  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',
+    ]);
+    $user->save();
+  }
+
 }
diff --git a/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php b/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php
new file mode 100644
index 0000000..720dccc
--- /dev/null
+++ b/core/modules/migrate_drupal/tests/src/Kernel/d7/MigrateDrupal7AuditIdsTest.php
@@ -0,0 +1,201 @@
+<?php
+
+namespace Drupal\Tests\migrate_drupal\Kernel\d7;
+
+use Drupal\aggregator\Entity\Feed;
+use Drupal\aggregator\Entity\Item;
+use Drupal\block_content\Entity\BlockContent;
+use Drupal\comment\Entity\Comment;
+use Drupal\file\Entity\File;
+use Drupal\menu_link_content\Entity\MenuLinkContent;
+use Drupal\KernelTests\FileSystemModuleDiscoveryDataProviderTrait;
+use Drupal\node\Entity\Node;
+use Drupal\taxonomy\Entity\Term;
+use Drupal\user\Entity\User;
+
+/**
+ * Tests the migration auditor for ID conflicts.
+ *
+ * @group migrate_drupal
+ */
+class MigrateDrupal7AuditIdsTest extends MigrateDrupal7TestBase {
+
+  use FileSystemModuleDiscoveryDataProviderTrait;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    // Enable all modules.
+    self::$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.
+    $conflicts = $this->container->get('migrate.id_auditor')->auditIds([
+      $this->getMigration('d7_node:page'),
+      $this->getMigration('d7_node:article'),
+    ]);
+    $this->assertTrue(empty($conflicts));
+  }
+
+  /**
+   * Tests all migrations with no ID conflicts.
+   */
+  public function testAllMigrationsWithNoIdConflicts() {
+    $plugin_manager = $this->container->get('plugin.manager.migration');
+
+    // Get all migration definitions.
+    $migrations = [];
+    foreach ($plugin_manager->getDefinitions() as $migration_id => $migration) {
+      $migrations[$migration_id] = $this->getMigration($migration_id);
+    }
+
+    // Audit the IDs of all migrations. There should be no conflicts since no
+    // content has been created.
+    $conflicts = $this->container->get('migrate.id_auditor')->auditIds($migrations);
+    $this->assertTrue(empty($conflicts));
+  }
+
+  /**
+   * Tests all migrations with ID conflicts.
+   */
+  public function testAllMigrationsWithIdConflicts() {
+    $plugin_manager = $this->container->get('plugin.manager.migration');
+
+    // Get all migration definitions.
+    $migrations = [];
+    foreach ($plugin_manager->getDefinitions() as $migration_id => $migration) {
+      $migrations[$migration_id] = $this->getMigration($migration_id);
+    }
+
+    // Create content.
+    $this->createContent();
+
+    // Audit the IDs of all migrations. There should be no conflicts since no
+    // content has been created.
+    $conflicts = $this->container->get('migrate.id_auditor')->auditIds($migrations);
+    ksort($conflicts);
+
+    $expected = [
+      'aggregator_feed',
+      'aggregator_item',
+      'block_content',
+      'comment',
+      'file',
+      'menu_link_content',
+      'node',
+      'taxonomy_term',
+      'user',
+    ];
+
+    $this->assertSame($expected, array_keys($conflicts));
+  }
+
+  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();
+  }
+
+}