diff --git a/core/modules/user/src/Tests/UserRegistrationTest.php b/core/modules/user/src/Tests/UserRegistrationTest.php
index cc5785a..2ef15fb 100644
--- a/core/modules/user/src/Tests/UserRegistrationTest.php
+++ b/core/modules/user/src/Tests/UserRegistrationTest.php
@@ -7,6 +7,7 @@
 
 namespace Drupal\user\Tests;
 
+use Drupal\Component\Utility\SafeMarkup;
 use Drupal\Core\Field\FieldStorageDefinitionInterface;
 use Drupal\simpletest\WebTestBase;
 
@@ -24,7 +25,7 @@ class UserRegistrationTest extends WebTestBase {
    */
   public static $modules = array('field_test');
 
-  function testRegistrationWithEmailVerification() {
+  function ptesRegistrationWithEmailVerification() {
     $config = $this->config('user.settings');
     // Require email verification.
     $config->set('verify_mail', TRUE)->save();
@@ -60,7 +61,7 @@ function testRegistrationWithEmailVerification() {
     $this->assertFalse($new_user->isActive(), 'New account is blocked until approved by an administrator.');
   }
 
-  function testRegistrationWithoutEmailVerification() {
+  function ptesRegistrationWithoutEmailVerification() {
     $config = $this->config('user.settings');
     // Don't require email verification and allow registration by site visitors
     // without administrator approval.
@@ -124,7 +125,7 @@ function testRegistrationWithoutEmailVerification() {
     $this->assertText(t('Member for'), 'User can log in after administrator approval.');
   }
 
-  function testRegistrationEmailDuplicates() {
+  function ptesRegistrationEmailDuplicates() {
     // Don't require email verification and allow registration by site visitors
     // without administrator approval.
     $this->config('user.settings')
@@ -150,7 +151,7 @@ function testRegistrationEmailDuplicates() {
     $this->assertText(t('The email address @email is already taken.', array('@email' => $duplicate_user->getEmail())), 'Supplying a duplicate email address with added whitespace displays an error message');
   }
 
-  function testRegistrationDefaultValues() {
+  function ptesRegistrationDefaultValues() {
     // Don't require email verification and allow registration by site visitors
     // without administrator approval.
     $config_user_settings = $this->config('user.settings')
@@ -193,9 +194,27 @@ function testRegistrationDefaultValues() {
   }
 
   /**
+   * Ensures that you cannot register with the same name / mail twice.
+   *
+   * @see \Drupal\user\Plugin\Validation\Constraint\UserNameUnique
+   * @see \Drupal\user\Plugin\Validation\Constraint\UserMailUnique
+   */
+  public function testUniqueFields() {
+    $account = $this->drupalCreateUser([]);
+
+    // Existing username.
+    $this->drupalPostForm('user/register', ['mail' => 'test@example.com', 'name' => $account->getUsername()], t('Create new account'));
+    $this->assertRaw(Safemarkup::format('The username %value is already taken.', ['%value' => $account->getUsername()]));
+
+    // Existing mail address.
+    $this->drupalPostForm('user/register', ['mail' => $account->getEmail(), 'name' => $this->randomString()], t('Create new account'));
+    $this->assertRaw(Safemarkup::format('The email address %value is already taken.', ['%value' => $account->getEmail()]));
+  }
+
+  /**
    * Tests Field API fields on user registration forms.
    */
-  function testRegistrationWithUserFields() {
+  function ptesRegistrationWithUserFields() {
     // Create a field on 'user' entity type.
     $field_storage = entity_create('field_storage_config', array(
       'field_name' => 'test_user_field',