Issue #2405023 by benjy: Cannot create base_field_override entity, ID already exists

core/modules/migrate/src/Plugin/migrate/destination/Entity.php

@@ -106,7 +106,7 @@ abstract class Entity extends DestinationBase implements ContainerFactoryPluginI
    *   The entity we're importing into.
    */
   protected function getEntity(Row $row, array $old_destination_id_values) {
-    $entity_id = $old_destination_id_values ? reset($old_destination_id_values) : $row->getDestinationProperty($this->getKey('id'));
+    $entity_id = $old_destination_id_values ? reset($old_destination_id_values) : $this->getEntityId($row);
     if (!empty($entity_id) && ($entity = $this->storage->load($entity_id))) {
       $this->updateEntity($entity, $row);
     }
@@ -125,6 +125,18 @@ abstract class Entity extends DestinationBase implements ContainerFactoryPluginI
     return $entity;
   }
 
+  /**
+   * Get the entity id of the row.
+   *
+   * @param \Drupal\migrate\Row $row
+   *   The row of data.
+   * @return string
+   *   The entity id for the row we're importing.
+   */
+  protected function getEntityId(Row $row) {
+    return $row->getDestinationProperty($this->getKey('id'));
+  }
+
   /**
    * Returns a specific entity key.
    *

core/modules/migrate/src/Plugin/migrate/destination/EntityBaseFieldOverride.php

@@ -0,0 +1,29 @@
+<?php
+
+/**
+ * @file
+ * Contains \Drupal\migrate\Plugin\migrate\destination\EntityBaseFieldOverride
+ */
+
+namespace Drupal\migrate\Plugin\migrate\destination;
+
+use Drupal\migrate\Row;
+
+/**
+ * @MigrateDestination(
+ *   id = "entity:base_field_override"
+ * )
+ */
+class EntityBaseFieldOverride extends EntityConfigBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function getEntityId(Row $row) {
+    $entity_type = $row->getDestinationProperty('entity_type');
+    $bundle = $row->getDestinationProperty('bundle');
+    $field_name = $row->getDestinationProperty('field_name');
+    return "$entity_type.$bundle.$field_name";
+  }
+
+}

core/modules/migrate_drupal/src/Tests/d6/MigrateNodeBundleSettingsTest.php

@@ -44,6 +44,9 @@ class MigrateNodeBundleSettingsTest extends MigrateDrupalTestBase {
     entity_create('node_type', array('type' => 'event'))->save();
     entity_create('node_type', array('type' => 'book'))->save();
 
+    // Create a config entity that already exists.
+    entity_create('base_field_override', array('field_name' => 'promote', 'entity_type' => 'node', 'bundle' => 'page',))->save();
+
     $id_mappings = array(
       'd6_node_type' => array(
         array(array('test_page'), array('test_page')),