diff --git a/core/lib/Drupal/Core/Field/FieldConfigInterface.php b/core/lib/Drupal/Core/Field/FieldConfigInterface.php
index 13ac848..f5a86a6 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', []);
+   *   $fields['field_username']->addConstraint('UniqueField', ['message' => t('The username %value is already taken.')]);
    * @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 c917cc9..da58934 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 = 'The @entity_type with the @field_name %value already exists.';
+  public $message = 'A @entity_type with the @field_name %value already exists.';
 
   /**
    * {@inheritdoc}
diff --git a/core/modules/block_content/src/Entity/BlockContent.php b/core/modules/block_content/src/Entity/BlockContent.php
index 40c19e9..ec57d5b 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('UniqueField');
+      ->addConstraint('UniqueField', ['message' => t('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/Tests/BlockContentCreationTest.php b/core/modules/block_content/src/Tests/BlockContentCreationTest.php
index 66c259a..7afb177 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('The custom block with the block description %value already exists.', array(
+    $this->assertRaw(format_string('A block with description %value already exists.', array(
       '%value' => $edit['info[0][value]']
     )));
     $this->assertResponse(200);
@@ -124,7 +124,7 @@ public function testBlockContentCreationMultipleViewModes() {
     $this->drupalPostForm('block/add/basic', $edit, t('Save'));
 
     // Check that the Basic block has been created.
-    $this->assertRaw(format_string('The custom block with the block description %value already exists.', array(
+    $this->assertRaw(format_string('A block with 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 e13f010..85bf6e9 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('The custom block with the block description %value already exists.', [
+    $this->assertEqual($violations[0]->getMessage(), format_string('A block with 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 2952bce..7e0b0de 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(),
-        'UniqueField' => array(),
+        'UniqueField' => array('message' => t('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('UniqueField')
+      ->addConstraint('UniqueField', ['message' => t('The email address %value is already taken.')])
       ->addConstraint('UserMailRequired')
       ->addConstraint('ProtectedUserField');
 
diff --git a/core/modules/user/src/Tests/UserEditTest.php b/core/modules/user/src/Tests/UserEditTest.php
index 8883381..46502f4 100644
--- a/core/modules/user/src/Tests/UserEditTest.php
+++ b/core/modules/user/src/Tests/UserEditTest.php
@@ -28,7 +28,7 @@ function testUserEdit() {
     // Test that error message appears when attempting to use a non-unique user name.
     $edit['name'] = $user2->getUsername();
     $this->drupalPostForm("user/" . $user1->id() . "/edit", $edit, t('Save'));
-    $this->assertRaw(t('The user with the name %name already exists.', array('%name' => $edit['name'])));
+    $this->assertRaw(t('The username %name is already taken.', array('%name' => $edit['name'])));
 
     // Check that filling out a single password field does not validate.
     $edit = array();
diff --git a/core/modules/user/src/Tests/UserRegistrationTest.php b/core/modules/user/src/Tests/UserRegistrationTest.php
index e7ee6f3..ff3fa30 100644
--- a/core/modules/user/src/Tests/UserRegistrationTest.php
+++ b/core/modules/user/src/Tests/UserRegistrationTest.php
@@ -141,13 +141,13 @@ function testRegistrationEmailDuplicates() {
 
     // Attempt to create a new account using an existing email address.
     $this->drupalPostForm('user/register', $edit, t('Create new account'));
-    $this->assertText(t('The user with the email @email already exists.', array('@email' => $duplicate_user->getEmail())), 'Supplying an exact duplicate email address displays an error message');
+    $this->assertText(t('The email address @email is already taken.', array('@email' => $duplicate_user->getEmail())), 'Supplying an exact duplicate email address displays an error message');
 
     // Attempt to bypass duplicate email registration validation by adding spaces.
     $edit['mail'] = '   ' . $duplicate_user->getEmail() . '   ';
 
     $this->drupalPostForm('user/register', $edit, t('Create new account'));
-    $this->assertText(t('The user with the email @email already exists.', array('@email' => $duplicate_user->getEmail())), 'Supplying a duplicate email address with added whitespace displays an error message');
+    $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() {
diff --git a/core/modules/user/src/Tests/UserValidationTest.php b/core/modules/user/src/Tests/UserValidationTest.php
index c6512f1..ec44a79 100644
--- a/core/modules/user/src/Tests/UserValidationTest.php
+++ b/core/modules/user/src/Tests/UserValidationTest.php
@@ -102,7 +102,7 @@ function testValidation() {
     $violations = $user->validate();
     $this->assertEqual(count($violations), 1, 'Violation found on name collision.');
     $this->assertEqual($violations[0]->getPropertyPath(), 'name');
-    $this->assertEqual($violations[0]->getMessage(), t('The user with the name %name already exists.', array('%name' => 'existing')));
+    $this->assertEqual($violations[0]->getMessage(), t('The username %name is already taken.', array('%name' => 'existing')));
 
     // Make the name valid.
     $user->set('name', $this->randomMachineName());
@@ -131,7 +131,7 @@ function testValidation() {
     $violations = $user->validate();
     $this->assertEqual(count($violations), 1, 'Violation found when email already exists.');
     $this->assertEqual($violations[0]->getPropertyPath(), 'mail');
-    $this->assertEqual($violations[0]->getMessage(), t('The user with the email %mail already exists.', array('%mail' => 'existing@example.com')));
+    $this->assertEqual($violations[0]->getMessage(), t('The email address %mail is already taken.', array('%mail' => 'existing@example.com')));
     $user->set('mail', NULL);
     $violations = $user->validate();
     $this->assertEqual(count($violations), 1, 'E-mail addresses may not be removed');