diff --git a/includes/form.inc b/includes/form.inc
index 8f16877..58274b0 100644
--- a/includes/form.inc
+++ b/includes/form.inc
@@ -850,7 +850,10 @@ 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, do not break HTML IDs of other forms.
+      drupal_static_reset('drupal_html_id');
+    }
 
     if ($form_state['submitted'] && !form_get_errors() && !$form_state['rebuild']) {
       // Execute form submit handlers.
diff --git a/modules/simpletest/tests/form.test b/modules/simpletest/tests/form.test
index 675e8d1..620621a 100644
--- a/modules/simpletest/tests/form.test
+++ b/modules/simpletest/tests/form.test
@@ -1750,3 +1750,34 @@ class FormCheckboxTestCase extends DrupalWebTestCase {
     }
   }
 }
+
+/**
+ * Tests uniqueness of generated HTML IDs.
+ */
+class HTMLIdTestCase extends DrupalWebTestCase {
+
+  public static function getInfo() {
+    return array(
+      'name' => 'Unique HTML IDs',
+      'description' => 'Tests functionality of drupal_html_id().',
+      'group' => 'Form API',
+    );
+  }
+
+  function setUp() {
+    parent::setUp('form_test');
+  }
+
+  /**
+   * Tests that HTML IDs do not get duplicated when form validation fails.
+   */
+  function testHTMLId() {
+    $this->drupalGet('form-test/double-form');
+    $this->assertNoDuplicateIds('There are no duplicate IDs');
+
+    // Submit second 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/modules/simpletest/tests/form_test.module b/modules/simpletest/tests/form_test.module
index 5d65276..6af18c6 100644
--- a/modules/simpletest/tests/form_test.module
+++ b/modules/simpletest/tests/form_test.module
@@ -194,6 +194,12 @@ 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,
+  );
 
   $items['form-test/load-include-menu'] = array(
     'title' => 'FAPI test loading includes',
@@ -1787,3 +1793,29 @@ function form_test_checkboxes_zero($form, &$form_state, $json = TRUE) {
 function _form_test_checkboxes_zero_no_redirect($form, &$form_state) {
   $form_state['redirect'] = FALSE;
 }
+
+/**
+ * Menu callback returns two instances of the same form.
+ */
+function form_test_double_form() {
+  return array(
+    'form1' => drupal_get_form('form_test_html_id'),
+    'form2' => drupal_get_form('form_test_html_id'),
+  );
+}
+
+/**
+ * Builds a simple form to test duplicate HTML IDs.
+ */
+function form_test_html_id($form, &$form_state) {
+  $form['name'] = array(
+    '#type' => 'textfield',
+    '#title' => 'name',
+    '#required' => TRUE,
+  );
+  $form['submit'] = array(
+    '#type' => 'submit',
+    '#value' => 'Save',
+  );
+  return $form;
+}