diff --git a/core/modules/user/src/AccountForm.php b/core/modules/user/src/AccountForm.php
index 0bcfad9..3a2b57d 100644
--- a/core/modules/user/src/AccountForm.php
+++ b/core/modules/user/src/AccountForm.php
@@ -407,4 +407,49 @@ public function submitForm(array &$form, FormStateInterface $form_state) {
       unset($_SESSION['pass_reset_'. $user->id()]);
     }
   }
+
+  /**
+   * {@inheritdoc}
+   */
+  public function validateForm(array &$form, FormStateInterface $form_state) {
+    $name_taken = FALSE;
+    $mail_taken = FALSE;
+
+    // For new registrations, make sure the username does not conflict with
+    // an existing user's email address.
+    $name_taken = (bool) db_select('users_field_data', 'ufd')
+      ->condition(
+        db_or()
+        ->condition('ufd.name', db_like($form_state->getValue('name')), 'LIKE')
+        ->condition('ufd.mail', db_like($form_state->getValue('name')), 'LIKE')
+      )
+      ->condition('ufd.status', 1)
+      ->range(0, 1)
+      ->countQuery()
+      ->execute()
+      ->fetchField();
+
+    if ($name_taken) {
+      $form_state->setErrorByName('name', $this->t('The name @name is already taken.', array('@name' => $form_state->getValue('name'))));
+    }
+
+    // For new registrations, make sure the email address does not conflict
+    // with an existing user's username.
+    $mail_taken = (bool) db_select('users_field_data', 'ufd')
+      ->condition(
+        db_or()
+        ->condition('ufd.mail', db_like($form_state->getValue('mail')), 'LIKE')
+        ->condition('ufd.name', db_like($form_state->getValue('mail')), 'LIKE')
+      )
+      ->condition('ufd.status', 1)
+      ->range(0, 1)
+      ->countQuery()
+      ->execute()
+      ->fetchField();
+
+    if ($mail_taken) {
+      $form_state->setErrorByName('mail', $this->t('The email address @email is already registered.', array('@email' => $form_state->getValue('mail'))));
+    }
+  }
+
 }
diff --git a/core/modules/user/src/Tests/UserRegistrationTest.php b/core/modules/user/src/Tests/UserRegistrationTest.php
index 78a5f57..927f40f 100644
--- a/core/modules/user/src/Tests/UserRegistrationTest.php
+++ b/core/modules/user/src/Tests/UserRegistrationTest.php
@@ -188,7 +188,7 @@ function testRegistrationConflicts() {
     // Attempt to create a new account using an email that matches an existing
     // username.
     $this->drupalPostForm('user/register', $edit, t('Create new account'));
-    $this->assertText(t('The e-mail address @email is already registered.', array('@email' => $edit['mail'])), "A user cannot be created when their email address matches an existing username.");
+    $this->assertText(t('The email address @email is already registered.', array('@email' => $edit['mail'])), "A user cannot be created when their email address matches an existing username.");
   }
 
   function testRegistrationDefaultValues() {