diff -u b/core/lib/Drupal/Core/Entity/EntityInterface.php b/core/lib/Drupal/Core/Entity/EntityInterface.php
--- b/core/lib/Drupal/Core/Entity/EntityInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityInterface.php
@@ -215,6 +215,8 @@
    * Creates a duplicate of the entity.
    *
    * @return static
+   *   A clone of $this with all identifiers unset, so saving it inserts a new
+   *   entity into the storage system.
    */
   public function createDuplicate();