diff --git a/core/lib/Drupal/Core/Field/FieldConfigInterface.php b/core/lib/Drupal/Core/Field/FieldConfigInterface.php
index 11da0df..13ac848 100644
--- a/core/lib/Drupal/Core/Field/FieldConfigInterface.php
+++ b/core/lib/Drupal/Core/Field/FieldConfigInterface.php
@@ -178,7 +178,7 @@ public function addPropertyConstraints($name, array $constraints);
    * @code
    *   // Add a constraint to the 'field_username' FieldItemList.
    *   // e.g. $node->field_username
-   *   $fields['field_username']->addConstraint('UserNameUnique', []);
+   *   $fields['field_username']->addConstraint('UniqueField', []);
    * @endcode
    *
    * If you wish to apply a constraint to a \Drupal\Core\Field\FieldItem instead
diff --git a/core/modules/aggregator/src/Entity/Feed.php b/core/modules/aggregator/src/Entity/Feed.php
index a5ed081..a442005 100644
--- a/core/modules/aggregator/src/Entity/Feed.php
+++ b/core/modules/aggregator/src/Entity/Feed.php
@@ -151,7 +151,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
         'weight' => -5,
       ))
       ->setDisplayConfigurable('form', TRUE)
-      ->addConstraint('FeedTitle', []);
+      ->addConstraint('UniqueField', ['message' => 'A feed named %value already exists. Enter a unique title.']);
 
     $fields['langcode'] = BaseFieldDefinition::create('language')
       ->setLabel(t('Language code'))
@@ -173,7 +173,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
         'weight' => -3,
       ))
       ->setDisplayConfigurable('form', TRUE)
-      ->addConstraint('FeedUrl', []);
+      ->addConstraint('UniqueField', ['message' => 'A feed with this URL %value already exists. Enter a unique URL.']);
 
     $intervals = array(900, 1800, 3600, 7200, 10800, 21600, 32400, 43200, 64800, 86400, 172800, 259200, 604800, 1209600, 2419200);
     $period = array_map(array(\Drupal::service('date.formatter'), 'formatInterval'), array_combine($intervals, $intervals));
diff --git a/core/modules/aggregator/src/Plugin/Validation/Constraint/FeedTitleConstraint.php b/core/modules/aggregator/src/Plugin/Validation/Constraint/FeedTitleConstraint.php
deleted file mode 100644
index 774d4e3..0000000
--- a/core/modules/aggregator/src/Plugin/Validation/Constraint/FeedTitleConstraint.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\aggregator\Plugin\Validation\Constraint\FeedTitleConstraint.
- */
-
-namespace Drupal\aggregator\Plugin\Validation\Constraint;
-
-use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
-
-/**
- * Supports validating feed titles.
- *
- * @Plugin(
- *   id = "FeedTitle",
- *   label = @Translation("Feed title", context = "Validation")
- * )
- */
-class FeedTitleConstraint extends UniqueFieldConstraint {
-
-  public $message = 'A feed named %value already exists. Enter a unique title.';
-
-}
diff --git a/core/modules/aggregator/src/Plugin/Validation/Constraint/FeedUrlConstraint.php b/core/modules/aggregator/src/Plugin/Validation/Constraint/FeedUrlConstraint.php
deleted file mode 100644
index ef2fec5..0000000
--- a/core/modules/aggregator/src/Plugin/Validation/Constraint/FeedUrlConstraint.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\aggregator\Plugin\Validation\Constraint\FeedUrlConstraint.
- */
-
-namespace Drupal\aggregator\Plugin\Validation\Constraint;
-
-use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
-
-/**
- * Supports validating feed URLs.
- *
- * @Plugin(
- *   id = "FeedUrl",
- *   label = @Translation("Feed URL", context = "Validation")
- * )
- */
-class FeedUrlConstraint extends UniqueFieldConstraint {
-
-  public $message = 'A feed with this URL %value already exists. Enter a unique URL.';
-
-}
diff --git a/core/modules/block_content/src/Entity/BlockContent.php b/core/modules/block_content/src/Entity/BlockContent.php
index f82c2d5..37b343d 100644
--- a/core/modules/block_content/src/Entity/BlockContent.php
+++ b/core/modules/block_content/src/Entity/BlockContent.php
@@ -188,7 +188,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
         'weight' => -5,
       ))
       ->setDisplayConfigurable('form', TRUE)
-      ->addConstraint('BlockContentDescription', []);
+      ->addConstraint('UniqueField', ['message' => 'A block with description %value already exists.']);
 
     $fields['type'] = BaseFieldDefinition::create('entity_reference')
       ->setLabel(t('Block type'))
diff --git a/core/modules/block_content/src/Plugin/Validation/Constraint/BlockContentDescriptionConstraint.php b/core/modules/block_content/src/Plugin/Validation/Constraint/BlockContentDescriptionConstraint.php
deleted file mode 100644
index 322bda6..0000000
--- a/core/modules/block_content/src/Plugin/Validation/Constraint/BlockContentDescriptionConstraint.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\block_content\Plugin\Validation\Constraint\BlockContentDescriptionConstraint.
- */
-
-namespace Drupal\block_content\Plugin\Validation\Constraint;
-
-use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
-
-/**
- * Supports validating uniqueness of block descriptions.
- *
- * @Plugin(
- *   id = "BlockContentDescription",
- *   label = @Translation("Block description unique", context = "Validation")
- * )
- */
-class BlockContentDescriptionConstraint extends UniqueFieldConstraint {
-
-  public $message = 'A block with description %value already exists.';
-
-}
diff --git a/core/modules/user/src/Entity/User.php b/core/modules/user/src/Entity/User.php
index fa4db6f..2db5d51 100644
--- a/core/modules/user/src/Entity/User.php
+++ b/core/modules/user/src/Entity/User.php
@@ -465,7 +465,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
         // No Length constraint here because the UserName constraint also covers
         // that.
         'UserName' => array(),
-        'UserNameUnique' => array(),
+        'UniqueField' => array('message' => 'The username %value is already taken.'),
       ));
 
     $fields['pass'] = BaseFieldDefinition::create('password')
@@ -477,7 +477,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
       ->setLabel(t('Email'))
       ->setDescription(t('The email of this user.'))
       ->setDefaultValue('')
-      ->addConstraint('UserMailUnique')
+      ->addConstraint('UniqueField', ['message' => 'The email address %value is already taken.'])
       ->addConstraint('UserMailRequired')
       ->addConstraint('ProtectedUserField');
 
diff --git a/core/modules/user/src/Plugin/Validation/Constraint/UserMailUnique.php b/core/modules/user/src/Plugin/Validation/Constraint/UserMailUnique.php
deleted file mode 100644
index b72e058..0000000
--- a/core/modules/user/src/Plugin/Validation/Constraint/UserMailUnique.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\user\Plugin\Validation\Constraint\UserMailUnique.
- */
-
-namespace Drupal\user\Plugin\Validation\Constraint;
-
-use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
-
-/**
- * Checks if a user's email address is unique on the site.
- *
- * @Plugin(
- *   id = "UserMailUnique",
- *   label = @Translation("User email unique", context = "Validation")
- * )
- */
-class UserMailUnique extends UniqueFieldConstraint {
-
-  public $message = 'The email address %value is already taken.';
-
-}
diff --git a/core/modules/user/src/Plugin/Validation/Constraint/UserNameUnique.php b/core/modules/user/src/Plugin/Validation/Constraint/UserNameUnique.php
deleted file mode 100644
index 6dbe431..0000000
--- a/core/modules/user/src/Plugin/Validation/Constraint/UserNameUnique.php
+++ /dev/null
@@ -1,24 +0,0 @@
-<?php
-
-/**
- * @file
- * Contains \Drupal\user\Plugin\Validation\Constraint\UserNameUnique.
- */
-
-namespace Drupal\user\Plugin\Validation\Constraint;
-
-use Drupal\Core\Validation\Plugin\Validation\Constraint\UniqueFieldConstraint;
-
-/**
- * Checks if a user name is unique on the site.
- *
- * @Plugin(
- *   id = "UserNameUnique",
- *   label = @Translation("User name unique", context = "Validation"),
- * )
- */
-class UserNameUnique extends UniqueFieldConstraint {
-
-  public $message = 'The username %value is already taken.';
-
-}
diff --git a/core/modules/user/src/Tests/UserCreateTest.php b/core/modules/user/src/Tests/UserCreateTest.php
index c906267..7a45a05 100644
--- a/core/modules/user/src/Tests/UserCreateTest.php
+++ b/core/modules/user/src/Tests/UserCreateTest.php
@@ -115,4 +115,22 @@ public function testUserAdd() {
       $this->assertEqual($user->isActive(), 'User is not blocked');
     }
   }
+
+  /**
+   * Tests the unique user email constraint.
+   */
+  public function testDuplicateUserEmail() {
+    $user1 = $this->drupalCreateUser();
+    $user1->setEmail('test_email@example.com')->save();
+    $violations = $user1->validate();
+    $this->assertEqual(0, $violations->count());
+
+    $user2 = $this->drupalCreateUser();
+    $user2->setEmail('test_email@example.com');
+    $violations = $user2->validate();
+    $this->assertEqual(1, $violations->count());
+    $this->assertEqual('The email address <em class="placeholder">test_email@example.com</em> is already taken.', $violations->get(0)->getMessage());
+    $this->assertEqual('mail', $violations->get(0)->getPropertyPath());
+  }
+
 }