diff --git a/core/lib/Drupal/Core/Field/FieldConfigInterface.php b/core/lib/Drupal/Core/Field/FieldConfigInterface.php
index f5a86a6..79486d8 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('UniqueField', ['message' => t('The username %value is already taken.')]);
+   *   $fields['field_username']->addConstraint('UniqueField');
    * @endcode
    *
    * If you wish to apply a constraint to a \Drupal\Core\Field\FieldItem instead
diff --git a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php
index da58934..770b375 100644
--- a/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php
+++ b/core/lib/Drupal/Core/Validation/Plugin/Validation/Constraint/UniqueFieldConstraint.php
@@ -19,7 +19,7 @@
  */
 class UniqueFieldConstraint extends Constraint {
 
-  public $message = 'A @entity_type with the @field_name %value already exists.';
+  public $message = 'A @entity_type with @field_name %value already exists.';
 
   /**
    * {@inheritdoc}
diff --git a/core/modules/aggregator/src/Entity/Feed.php b/core/modules/aggregator/src/Entity/Feed.php
index fc1b9ee..68effd4 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('UniqueField', ['message' => t('A feed named %value already exists. Enter a unique title.')]);
+      ->addConstraint('FeedTitle');
 
     $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('UniqueField', ['message' => t('A feed with this URL %value already exists. Enter a unique URL.')]);
+      ->addConstraint('FeedUrl');
 
     $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/block_content/src/Entity/BlockContent.php b/core/modules/block_content/src/Entity/BlockContent.php
index 8680ffc..f679aba 100644
--- a/core/modules/block_content/src/Entity/BlockContent.php
+++ b/core/modules/block_content/src/Entity/BlockContent.php
@@ -190,7 +190,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
         'weight' => -5,
       ))
       ->setDisplayConfigurable('form', TRUE)
-      ->addConstraint('UniqueField', ['message' => t('A block with description %value already exists.')]);
+      ->addConstraint('UniqueField');
 
     $fields['type'] = BaseFieldDefinition::create('entity_reference')
       ->setLabel(t('Block type'))
diff --git a/core/modules/block_content/src/Tests/BlockContentCreationTest.php b/core/modules/block_content/src/Tests/BlockContentCreationTest.php
index 7afb177..321c451 100644
--- a/core/modules/block_content/src/Tests/BlockContentCreationTest.php
+++ b/core/modules/block_content/src/Tests/BlockContentCreationTest.php
@@ -66,7 +66,7 @@ public function testBlockContentCreation() {
     $this->drupalPostForm('block/add/basic', $edit, t('Save'));
 
     // Check that the Basic block has been created.
-    $this->assertRaw(format_string('A block with description %value already exists.', array(
+    $this->assertRaw(format_string('A custom block with block description %value already exists.', array(
       '%value' => $edit['info[0][value]']
     )));
     $this->assertResponse(200);
diff --git a/core/modules/block_content/src/Tests/BlockContentValidationTest.php b/core/modules/block_content/src/Tests/BlockContentValidationTest.php
index 85bf6e9..d5ebde2 100644
--- a/core/modules/block_content/src/Tests/BlockContentValidationTest.php
+++ b/core/modules/block_content/src/Tests/BlockContentValidationTest.php
@@ -37,7 +37,7 @@ public function testValidation() {
     // Make sure the violation is on the info property
     $this->assertEqual($violations[0]->getPropertyPath(), 'info');
     // Make sure the message is correct.
-    $this->assertEqual($violations[0]->getMessage(), format_string('A block with description %value already exists.', [
+    $this->assertEqual($violations[0]->getMessage(), format_string('A custom block with block description %value already exists.', [
       '%value' => $block->label(),
     ]));
   }
diff --git a/core/modules/user/src/Entity/User.php b/core/modules/user/src/Entity/User.php
index 03b0ba4..53155a0 100644
--- a/core/modules/user/src/Entity/User.php
+++ b/core/modules/user/src/Entity/User.php
@@ -468,7 +468,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
         // No Length constraint here because the UserName constraint also covers
         // that.
         'UserName' => array(),
-        'UniqueField' => array('message' => t('The username %value is already taken.')),
+        'UserNameUnique' => array(),
       ));
 
     $fields['pass'] = BaseFieldDefinition::create('password')
@@ -480,7 +480,7 @@ public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
       ->setLabel(t('Email'))
       ->setDescription(t('The email of this user.'))
       ->setDefaultValue('')
-      ->addConstraint('UniqueField', ['message' => t('The email address %value is already taken.')])
+      ->addConstraint('UserMailUnique')
       ->addConstraint('UserMailRequired')
       ->addConstraint('ProtectedUserField');