diff --git a/core/lib/Drupal/Core/Render/Element/MachineName.php b/core/lib/Drupal/Core/Render/Element/MachineName.php
index 66c85c7..d60d550 100644
--- a/core/lib/Drupal/Core/Render/Element/MachineName.php
+++ b/core/lib/Drupal/Core/Render/Element/MachineName.php
@@ -232,12 +232,38 @@ public static function validateMachineName(&$element, FormStateInterface $form_s
       }
     }
 
-    // Verify that the machine name is unique.
-    if ($element['#default_value'] !== $element['#value']) {
-      $function = $element['#machine_name']['exists'];
-      if (call_user_func($function, $element['#value'], $element, $form_state)) {
-        $form_state->setError($element, t('The machine-readable name is already in use. It must be unique.'));
+    // Verify that the machine name is unique. Check form state if we need to
+    // explicitly validate the element. This happens every time when an error
+    // has been set, since this breaks on AJAX requests.
+    // @see https://www.drupal.org/node/2557299.
+    $needs_revalidation = $form_state->get('needs_revalidation', $element['#name']);
+    if ($needs_revalidation) {
+      static::isUnique($element, $form_state);
+    }
+    elseif ($element['#default_value'] !== $element['#value']) {
+      static::isUnique($element, $form_state);
+    }
+
+  }
+
+  /**
+   * Verify that the machine name is unique.
+   *
+   * @param $element
+   *   The form element
+   * @param FormStateInterface $form_state
+   *   The form state interface.
+   * @param (optional) $set_needs_revalidation
+   *   Whether to set a property on form state in case of an error to revalidate
+   *   the element again on the next submission.
+   */
+  public static function isUnique($element, FormStateInterface $form_state, $set_needs_revalidation = TRUE) {
+    $function = $element['#machine_name']['exists'];
+    if (call_user_func($function, $element['#value'], $element, $form_state)) {
+      if ($set_needs_revalidation) {
+        $form_state->set('needs_revalidation', $element['#name']);
       }
+      $form_state->setError($element, t('The machine-readable name is already in use. It must be unique.'));
     }
   }
 
diff --git a/core/modules/ckeditor/src/Tests/CKEditorAdminTest.php b/core/modules/ckeditor/src/Tests/CKEditorAdminTest.php
index 6c5ee13..83a326d 100644
--- a/core/modules/ckeditor/src/Tests/CKEditorAdminTest.php
+++ b/core/modules/ckeditor/src/Tests/CKEditorAdminTest.php
@@ -218,6 +218,20 @@ function testExistingFormat() {
     $editor = entity_load('editor', 'filtered_html');
     $this->assertTrue($editor instanceof Editor, 'An Editor config entity exists.');
     $this->assertIdentical($expected_settings, $editor->getSettings());
+
+    $this->drupalGet('admin/config/content/formats/add');
+    // Now attempt to add another filter format with the same editor and same
+    // machine name.
+    $edit = array(
+      'format' => 'filtered_html',
+      'name' => 'Filtered HTML',
+      'editor[editor]' => 'ckeditor',
+    );
+    $this->drupalPostAjaxForm(NULL, $edit, 'editor_configure');
+    $this->assertResponse(200);
+    $this->drupalPostForm(NULL, $edit, t('Save configuration'));
+    $this->assertResponse(200);
+    $this->assertText(t('The machine-readable name is already in use. It must be unique.'));
   }
 
   /**