diff --git a/core/includes/form.inc b/core/includes/form.inc
index a555bf0..fea8eb5 100644
--- a/core/includes/form.inc
+++ b/core/includes/form.inc
@@ -856,7 +856,10 @@ function drupal_process_form($form_id, &$form, &$form_state) {
     // cache when a form is processed, so scenarios that result in
     // the form being built behind the scenes and again for the
     // browser don't increment all the element IDs needlessly.
-    drupal_static_reset('drupal_html_id');
+    if (!form_get_errors()) {
+      // In case of errors no rebuild happens so elements still the same.
+      drupal_static_reset('drupal_html_id');
+    }
 
     if ($form_state['submitted'] && !form_get_errors() && !$form_state['rebuild']) {
       // Execute form submit handlers.
diff --git a/core/modules/system/tests/form.test b/core/modules/system/tests/form.test
index fe5f922..294be66 100644
--- a/core/modules/system/tests/form.test
+++ b/core/modules/system/tests/form.test
@@ -2011,3 +2011,35 @@ class FormUrlTestCase extends WebTestBase {
     $this->assertEqual($values['url_required'], $edit['url_required']);
   }
 }
+
+/**
+ * Tests html id element unique.
+ */
+class FormHtmlIdTestCase extends WebTestBase {
+  protected $profile = 'testing';
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Form API html id',
+      'description' => 'Tests the form API html id unique.',
+      'group' => 'Form API',
+    );
+  }
+
+  public function setUp() {
+    parent::setUp('form_test');
+  }
+
+  /**
+   * Tests that #type 'url' fields are properly validated and trimmed.
+   */
+  function testHtmlId() {
+    // Tests that element ID's don't get duplicated when form validation fails.
+    $this->drupalGet('form-test/double-form');
+    // Submit second node form with empty title.
+    $edit = array();
+    $this->drupalPost(NULL, $edit, 'Save', array(), array(), 'form-test-html-id--2');
+    $this->assertNoDuplicateIds('There are no duplicate IDs');
+
+  }
+}
diff --git a/core/modules/system/tests/modules/form_test/form_test.module b/core/modules/system/tests/modules/form_test/form_test.module
index f681c75..3cc9b08 100644
--- a/core/modules/system/tests/modules/form_test/form_test.module
+++ b/core/modules/system/tests/modules/form_test/form_test.module
@@ -241,6 +241,12 @@ function form_test_menu() {
       'type' => MENU_CALLBACK,
     );
   }
+  $items['form-test/double-form'] = array(
+    'title' => 'Double form test',
+    'page callback' => 'form_test_double_form',
+    'access callback' => TRUE,
+    'type' => MENU_CALLBACK,
+  );
 
   $items['form-test/load-include-menu'] = array(
     'title' => 'FAPI test loading includes',
@@ -2164,3 +2170,35 @@ function form_test_required_attribute($form, &$form_state) {
 
   return $form;
 }
+
+/**
+ * Menu callback that returns two instances of the same form.
+ */
+function form_test_double_form() {
+  return array(
+    'form1' => drupal_get_form('form_test_html_id'),
+    'form2' => drupal_get_form('form_test_html_id', TRUE),
+  );
+}
+
+/**
+ * Builds a form to test html IDs.
+ */
+function form_test_html_id($form, &$form_state, $has_file_field = FALSE) {
+  $form['name'] = array(
+    '#type' => 'textfield',
+    '#title' => 'name',
+    '#required' => TRUE,
+  );
+  if ($has_file_field) {
+    $form['file'] = array(
+      '#type' => 'file',
+      '#title' => 'file',
+    );
+  }
+  $form['submit'] = array(
+    '#type' => 'submit',
+    '#value' => 'Save',
+  );
+  return $form;
+}