diff --git a/core/lib/Drupal/Core/Render/Element/MachineName.php b/core/lib/Drupal/Core/Render/Element/MachineName.php
index c1b1fa05c2..6e3ccb6b32 100644
--- a/core/lib/Drupal/Core/Render/Element/MachineName.php
+++ b/core/lib/Drupal/Core/Render/Element/MachineName.php
@@ -213,8 +213,7 @@ public static function processMachineName(&$element, FormStateInterface $form_st
   /**
    * Form element validation handler for machine_name elements.
    *
-   * Note that #maxlength is validated by the form validator already, in
-   * \Drupal\Core\Form\FormValidatorInterface::validateForm().
+   * Note that #maxlength is validated by _form_validate() already.
    *
    * This checks that the submitted value:
    * - Does not contain the replacement character only.
@@ -250,10 +249,12 @@ public static function validateMachineName(&$element, FormStateInterface $form_s
     // Verify that the machine name is unique. Check form state if we need to
     // explicitly validate the element. This happens every time when an error
     // has been set, since this breaks on AJAX requests
-    if ($form_state->get('needs_revalidation') === $element['#name'] || $element['#default_value'] !== $element['#value']) {
+    $needs_revalidating = $form_state->get('machine_name.needs_revalidating') ?: [];
+    if ($element['#default_value'] !== $element['#value'] || isset($needs_revalidating[$element['#name']])) {
       $function = $element['#machine_name']['exists'];
       if (call_user_func($function, $element['#value'], $element, $form_state)) {
-        $form_state->set('needs_revalidation', $element['#name']);
+        $needs_revalidating[$element['#name']] = TRUE;
+        $form_state->set('machine_name.needs_revalidating', $needs_revalidating);
         $form_state->setError($element, t('The machine-readable name is already in use. It must be unique.'));
       }
     }
diff --git a/core/modules/media/src/MediaTypeForm.php b/core/modules/media/src/MediaTypeForm.php
index cd01c13a1c..d696556016 100644
--- a/core/modules/media/src/MediaTypeForm.php
+++ b/core/modules/media/src/MediaTypeForm.php
@@ -229,18 +229,6 @@ public function form(array $form, FormStateInterface $form_state) {
     return $form;
   }
 
-  /**
-   * Form submission handler to rebuild the form on select submit.
-   *
-   * @param array $form
-   *   Full form array.
-   * @param \Drupal\Core\Form\FormStateInterface $form_state
-   *   Current form state.
-   */
-  public static function rebuildSubmit(array &$form, FormStateInterface $form_state) {
-    $form_state->setRebuild();
-  }
-
   /**
    * Prepares workflow options to be used in the 'checkboxes' form element.
    *
diff --git a/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php b/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php
index 2844410ed8..207b542f95 100644
--- a/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php
+++ b/core/modules/media/tests/src/FunctionalJavascript/MediaTypeCreationTest.php
@@ -99,9 +99,7 @@ public function testMediaTypeCreationReuseSourceField() {
 
     // Create a new media type, which should create a new field we can reuse.
     $this->drupalGet('/admin/structure/media/add');
-    // Choose the source plugin _before_ setting the label and machine name in
-    // order to guard against the regression in
-    // https://www.drupal.org/project/drupal/issues/2557299.
+    // Choose the source plugin before setting the label and machine name.
     $page->selectFieldOption('Media source', 'test');
     $result = $assert_session->waitForElementVisible('css', 'fieldset[data-drupal-selector="edit-source-configuration"]');
     $this->assertNotEmpty($result);
@@ -115,8 +113,7 @@ public function testMediaTypeCreationReuseSourceField() {
     $this->drupalGet('admin/structure/media/add');
 
     // Select the media source used by our media type. Do this before setting
-    // the label and machine name in order to guard against the regression in
-    // https://www.drupal.org/project/drupal/issues/2557299.
+    // the label and machine name.
     $assert_session->fieldExists('Media source');
     $assert_session->optionExists('Media source', 'test');
     $page->selectFieldOption('Media source', 'test');
diff --git a/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php b/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
index 04ecc0a521..ff2dcaac58 100644
--- a/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
+++ b/core/modules/system/tests/modules/form_test/src/Form/FormTestMachineNameValidationForm.php
@@ -21,12 +21,16 @@ public function getFormId() {
    * {@inheritdoc}
    */
   public function buildForm(array $form, FormStateInterface $form_state) {
+    // Disable client-side validation so that we can test AJAX requests with
+    // invalid input.
+    $form['#attributes']['novalidate'] = 'novalidate';
+
     $form['name'] = [
       '#type' => 'textfield',
       '#default_value' => $form_state->getValue('name'),
       '#maxlength' => 50,
       '#required' => TRUE,
-      '#title' => 'Name'
+      '#title' => 'Name',
     ];
 
     // The default value simulates how an entity form works, which has default
@@ -47,6 +51,20 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       ],
     ];
 
+    // Test support for multiple machine names on the form. Although this has
+    // the default value duplicate it should not generate an error because it
+    // is the default value.
+    $form['id2'] = [
+      '#type' => 'machine_name',
+      '#default_value' => 'duplicate',
+      '#maxlength' => 50,
+      '#required' => TRUE,
+      '#machine_name' => [
+        'exists' => [$this, 'load'],
+        'source' => ['name'],
+      ],
+    ];
+
     $form['snack'] = [
       '#type' => 'select',
       '#title' => $this->t('Select a snack'),
@@ -57,7 +75,6 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       ],
       '#required' => TRUE,
       '#ajax' => [
-        'trigger_as' => ['name' => 'snack_configure'],
         'callback' => '::buildAjaxSnackConfigureForm',
         'wrapper' => 'snack-config-form',
         'method' => 'replace',
@@ -71,29 +88,6 @@ public function buildForm(array $form, FormStateInterface $form_state) {
       ],
       '#tree' => TRUE,
     ];
-    $form['snack_configure_button'] = [
-      '#type' => 'submit',
-      '#name' => 'snack_configure',
-      '#value' => 'Configure snack',
-      '#limit_validation_errors' => [['snack']],
-      '#validate' => ['::buildAjaxSnackConfigureFormValidate'],
-      '#submit' => ['::buildAjaxSnackConfigureFormSubmit'],
-      '#executes_submit_callback' => TRUE,
-      '#ajax' => [
-        'callback' => '::buildAjaxSnackConfigureForm',
-        'wrapper' => 'snack-config-form',
-      ],
-      '#attributes' => ['class' => ['js-hide']],
-    ];
-    $form['snack_configs']['apple']['#type'] = 'details';
-    $form['snack_configs']['apple']['#title'] = 'Configure Apple';
-    $form['snack_configs']['apple']['#open'] = TRUE;
-    $form['snack_configs']['pear']['#type'] = 'details';
-    $form['snack_configs']['pear']['#title'] = 'Configure pear';
-    $form['snack_configs']['pear']['#open'] = TRUE;
-    $form['snack_configs']['potato']['#type'] = 'details';
-    $form['snack_configs']['potato']['#title'] = 'Configure potato';
-    $form['snack_configs']['potato']['#open'] = TRUE;
 
     $form['submit'] = [
       '#type' => 'submit',
@@ -130,7 +124,7 @@ public function buildAjaxSnackConfigureForm(array $form, FormStateInterface $for
    * @return bool
    */
   public function load($machine_name) {
-    if ($machine_name === 'duplicate') {
+    if (strpos($machine_name, 'duplicate') !== FALSE) {
       return TRUE;
     }
 
diff --git a/core/modules/system/tests/src/Functional/Form/FormTest.php b/core/modules/system/tests/src/Functional/Form/FormTest.php
index 1d9c6399d5..2407a3f51a 100644
--- a/core/modules/system/tests/src/Functional/Form/FormTest.php
+++ b/core/modules/system/tests/src/Functional/Form/FormTest.php
@@ -770,36 +770,4 @@ public function testRequiredAttribute() {
     $this->assertTrue(!empty($element), 'The textarea has the proper required attribute.');
   }
 
-  /**
-   * Tests that a machine name is still required after an AJAX submit.
-   *
-   * This protects against the regression in https://www.drupal.org/node/2557299.
-   */
-  public function testMachineNameRequiredFormAjaxSubmit() {
-    $this->drupalGet('/form-test/form-test-machine-name-validation');
-
-    $edit = [];
-    $this->submitForm($edit, 'Save');
-    $this->assertResponse(200);
-    $this->assertText('Machine-readable name field is required.');
-
-    $edit = [
-      'name' => 'test 1',
-      'id' => 'machine1',
-      'snack' => 'apple'
-    ];
-    $this->submitForm($edit, 'Save');
-    $this->assertResponse(200);
-    $this->assertText('The form_test_machine_name_validation_form form has been submitted successfully.');
-
-    $edit = [
-      'name' => 'test 2',
-      'id' => 'duplicate',
-      'snack' => 'potato',
-    ];
-    $this->submitForm($edit, 'snack_configure');
-    $this->submitForm($edit, 'Save');
-    $this->assertResponse(200);
-    $this->assertText('The machine-readable name is already in use. It must be unique.');
-  }
 }
diff --git a/core/modules/system/tests/src/FunctionalJavascript/MachineNameValidationTest.php b/core/modules/system/tests/src/FunctionalJavascript/MachineNameValidationTest.php
new file mode 100644
index 0000000000..e09ca6cf4c
--- /dev/null
+++ b/core/modules/system/tests/src/FunctionalJavascript/MachineNameValidationTest.php
@@ -0,0 +1,57 @@
+<?php
+
+namespace Drupal\Tests\system\FunctionalJavascript;
+
+use Drupal\FunctionalJavascriptTests\JavascriptTestBase;
+
+/**
+ * Tests validation of machine name elements.
+ *
+ * @group Form
+ */
+class MachineNameValidationTest extends JavascriptTestBase {
+
+  /**
+   * {@inheritdoc}
+   */
+  protected static $modules = ['form_test'];
+
+  /**
+   * Tests that a machine name is still required after an AJAX submit.
+   *
+   * This guards against the regression in https://www.drupal.org/node/2557299.
+   */
+  public function testMachineNameRequiredFormAjaxSubmit() {
+    $assert = $this->assertSession();
+    $this->drupalGet('/form-test/form-test-machine-name-validation');
+
+    // Test errors after with no AJAX.
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('Machine-readable name field is required.');
+    // Ensure only the first machine name field has an error.
+    $this->assertTrue($assert->fieldExists('id')->hasClass('error'));
+    $this->assertFalse($assert->fieldExists('id2')->hasClass('error'));
+
+    // Test a successful submit after using AJAX.
+    $assert->fieldExists('Name')->setValue('test 1');
+    $assert->fieldExists('id')->setValue('test_1');
+    $assert->selectExists('snack')->selectOption('apple');
+    $assert->assertWaitOnAjaxRequest();
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('The form_test_machine_name_validation_form form has been submitted successfully.');
+
+    // Test errors after using AJAX.
+    $assert->fieldExists('Name')->setValue('duplicate');
+    $this->assertJsCondition('document.forms[0].id.value === "duplicate"');
+    $assert->fieldExists('id2')->setValue('duplicate2');
+    $assert->selectExists('snack')->selectOption('potato');
+    $assert->assertWaitOnAjaxRequest();
+    $assert->buttonExists('Save')->press();
+    $assert->pageTextContains('The machine-readable name is already in use. It must be unique.');
+    // Ensure both machine name fields both have errors.
+    $this->assertTrue($assert->fieldExists('id')->hasClass('error'));
+    $this->assertTrue($assert->fieldExists('id2')->hasClass('error'));
+
+  }
+
+}