diff --git a/core/lib/Drupal/Core/Messenger/LegacyMessenger.php b/core/lib/Drupal/Core/Messenger/LegacyMessenger.php
index bdc66ffe39bc33fb5e7f73048cb0509b19e92cff..86034258315edf2adb043acf62417d11422fd17f 100644
--- a/core/lib/Drupal/Core/Messenger/LegacyMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/LegacyMessenger.php
@@ -34,7 +34,7 @@ class LegacyMessenger implements MessengerInterface {
    * {@inheritdoc}
    */
   public function addError($message, $repeat = FALSE) {
-    return $this->addMessage($message, static::TYPE_ERROR);
+    return $this->addMessage($message, static::TYPE_ERROR, $repeat);
   }
 
   /**
@@ -67,14 +67,14 @@ public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE)
    * {@inheritdoc}
    */
   public function addStatus($message, $repeat = FALSE) {
-    return $this->addMessage($message, static::TYPE_STATUS);
+    return $this->addMessage($message, static::TYPE_STATUS, $repeat);
   }
 
   /**
    * {@inheritdoc}
    */
   public function addWarning($message, $repeat = FALSE) {
-    return $this->addMessage($message, static::TYPE_WARNING);
+    return $this->addMessage($message, static::TYPE_WARNING, $repeat);
   }
 
   /**
@@ -100,13 +100,16 @@ protected function getMessengerService() {
     if (\Drupal::hasService('messenger')) {
       // Note: because the container has the potential to be rebuilt during
       // requests, this service cannot be directly stored on this class.
+      /** @var \Drupal\Core\Messenger\MessengerInterface $messenger */
       $messenger = \Drupal::service('messenger');
 
       // Transfer any messages into the service.
       if (isset($this->messages)) {
         foreach ($this->messages as $type => $messages) {
           foreach ($messages as $message) {
-            $messenger->addMessage($message, $type);
+            // Force repeat to TRUE since this is merging existing messages to
+            // the Messenger service and would have already checked this prior.
+            $messenger->addMessage($message, $type, TRUE);
           }
         }
         unset($this->messages);
diff --git a/core/lib/Drupal/Core/Messenger/Messenger.php b/core/lib/Drupal/Core/Messenger/Messenger.php
index b8b6c3ddf369d3fa41e74e0daa2a637805630f36..c0949438cc56db131a6f14a02413cf09ad25855e 100644
--- a/core/lib/Drupal/Core/Messenger/Messenger.php
+++ b/core/lib/Drupal/Core/Messenger/Messenger.php
@@ -43,7 +43,7 @@ public function __construct(FlashBagInterface $flash_bag, KillSwitch $killSwitch
    * {@inheritdoc}
    */
   public function addError($message, $repeat = FALSE) {
-    return $this->addMessage($message, static::TYPE_ERROR);
+    return $this->addMessage($message, static::TYPE_ERROR, $repeat);
   }
 
   /**
@@ -70,14 +70,14 @@ public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE)
    * {@inheritdoc}
    */
   public function addStatus($message, $repeat = FALSE) {
-    return $this->addMessage($message, static::TYPE_STATUS);
+    return $this->addMessage($message, static::TYPE_STATUS, $repeat);
   }
 
   /**
    * {@inheritdoc}
    */
   public function addWarning($message, $repeat = FALSE) {
-    return $this->addMessage($message, static::TYPE_WARNING);
+    return $this->addMessage($message, static::TYPE_WARNING, $repeat);
   }
 
   /**
diff --git a/core/tests/Drupal/KernelTests/Core/Messenger/MessengerLegacyTest.php b/core/tests/Drupal/KernelTests/Core/Messenger/MessengerLegacyTest.php
new file mode 100644
index 0000000000000000000000000000000000000000..29334647db175319cc1dd9e8d5d8da3539ea9921
--- /dev/null
+++ b/core/tests/Drupal/KernelTests/Core/Messenger/MessengerLegacyTest.php
@@ -0,0 +1,108 @@
+<?php
+
+namespace Drupal\KernelTests\Core\Messenger;
+
+use Drupal\Core\Messenger\LegacyMessenger;
+use Drupal\Core\Messenger\Messenger;
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\KernelTests\KernelTestBase;
+
+/**
+ * @group Messenger
+ * @coversDefaultClass \Drupal\Core\Messenger\LegacyMessenger
+ *
+ * Note: The Symphony PHPUnit Bridge automatically treats any test class that
+ * starts with "Legacy" as a deprecation. To subvert that, reverse it here.
+ *
+ * @see http://symfony.com/blog/new-in-symfony-2-7-phpunit-bridge
+ * @see https://www.drupal.org/node/2931598#comment-12395743
+ */
+class MessengerLegacyTest extends KernelTestBase {
+
+  /**
+   * Retrieves the Messenger service from LegacyMessenger.
+   *
+   * @param \Drupal\Core\Messenger\LegacyMessenger $legacy_messenger
+   *   The legacy messenger.
+   *
+   * @return \Drupal\Core\Messenger\MessengerInterface|null
+   *   A messenger implementation.
+   */
+  protected function getMessengerService(LegacyMessenger $legacy_messenger) {
+    $method = new \ReflectionMethod($legacy_messenger, 'getMessengerService');
+    $method->setAccessible(TRUE);
+    return $method->invoke($legacy_messenger);
+  }
+
+  /**
+   * @covers \Drupal::messenger
+   * @covers ::getMessengerService
+   * @covers ::all
+   * @covers ::addMessage
+   * @covers ::addError
+   * @covers ::addStatus
+   * @covers ::addWarning
+   */
+  public function testMessages() {
+    // Save the current container for later use.
+    $container = \Drupal::getContainer();
+
+    // Unset the container to mimic not having one.
+    \Drupal::unsetContainer();
+
+    /** @var \Drupal\Core\Messenger\LegacyMessenger $messenger */
+    // Verify that the Messenger service doesn't exists.
+    $messenger = \Drupal::messenger();
+    $this->assertNull($this->getMessengerService($messenger));
+
+    // Add messages.
+    $messenger->addMessage('Foobar', 'custom');
+    $messenger->addMessage('Foobar', 'custom', TRUE);
+    $messenger->addError('Foo');
+    $messenger->addError('Foo', TRUE);
+
+    // Verify that retrieving another instance and adding more messages works.
+    $messenger = \Drupal::messenger();
+    $messenger->addStatus('Bar');
+    $messenger->addStatus('Bar', TRUE);
+    $messenger->addWarning('Fiz');
+    $messenger->addWarning('Fiz', TRUE);
+
+    // Restore the container.
+    \Drupal::setContainer($container);
+
+    // Verify that the Messenger service exists.
+    $messenger = \Drupal::messenger();
+    $this->assertInstanceOf(Messenger::class, $this->getMessengerService($messenger));
+
+    // Add more messages.
+    $messenger->addMessage('Platypus', 'custom');
+    $messenger->addMessage('Platypus', 'custom', TRUE);
+    $messenger->addError('Rhinoceros');
+    $messenger->addError('Rhinoceros', TRUE);
+    $messenger->addStatus('Giraffe');
+    $messenger->addStatus('Giraffe', TRUE);
+    $messenger->addWarning('Cheetah');
+    $messenger->addWarning('Cheetah', TRUE);
+
+    // Verify all messages added via LegacyMessenger are accounted for.
+    $messages = $messenger->all();
+    $this->assertContains('Foobar', $messages['custom']);
+    $this->assertContains('Foo', $messages[MessengerInterface::TYPE_ERROR]);
+    $this->assertContains('Bar', $messages[MessengerInterface::TYPE_STATUS]);
+    $this->assertContains('Fiz', $messages[MessengerInterface::TYPE_WARNING]);
+
+    // Verify all messages added via Messenger service are accounted for.
+    $this->assertContains('Platypus', $messages['custom']);
+    $this->assertContains('Rhinoceros', $messages[MessengerInterface::TYPE_ERROR]);
+    $this->assertContains('Giraffe', $messages[MessengerInterface::TYPE_STATUS]);
+    $this->assertContains('Cheetah', $messages[MessengerInterface::TYPE_WARNING]);
+
+    // Verify repeat counts.
+    $this->assertCount(4, $messages['custom']);
+    $this->assertCount(4, $messages[MessengerInterface::TYPE_STATUS]);
+    $this->assertCount(4, $messages[MessengerInterface::TYPE_WARNING]);
+    $this->assertCount(4, $messages[MessengerInterface::TYPE_ERROR]);
+  }
+
+}
diff --git a/core/tests/Drupal/KernelTests/Core/Messenger/MessengerTest.php b/core/tests/Drupal/KernelTests/Core/Messenger/MessengerTest.php
index 362069756eae2c8f7b10dac5a95c17ae0b8321d0..32c4743d93d401d00bd3f51710a08342fe3e6a30 100644
--- a/core/tests/Drupal/KernelTests/Core/Messenger/MessengerTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Messenger/MessengerTest.php
@@ -2,83 +2,164 @@
 
 namespace Drupal\KernelTests\Core\Messenger;
 
-use Drupal\Core\Messenger\LegacyMessenger;
-use Drupal\Core\Messenger\Messenger;
 use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Core\Render\Markup;
+use Drupal\Core\StringTranslation\TranslatableMarkup;
 use Drupal\KernelTests\KernelTestBase;
 
 /**
  * @group Messenger
- * @coversDefaultClass \Drupal\Core\Messenger\LegacyMessenger
+ * @coversDefaultClass \Drupal\Core\Messenger\Messenger
  */
 class MessengerTest extends KernelTestBase {
 
   /**
-   * Retrieves the Messenger service from LegacyMessenger.
+   * The messenger under test.
    *
-   * @param \Drupal\Core\Messenger\LegacyMessenger $legacy_messenger
-   *
-   * @return \Drupal\Core\Messenger\MessengerInterface|null
+   * @var \Drupal\Core\Messenger\MessengerInterface
+   */
+  protected $messenger;
+
+  /**
+   * {@inheritdoc}
+   */
+  protected function setUp() {
+    parent::setUp();
+    $this->messenger = \Drupal::service('messenger');
+  }
+
+  /**
+   * @covers ::addStatus
+   * @covers ::deleteByType
+   * @covers ::messagesByType
    */
-  protected function getMessengerService(LegacyMessenger $legacy_messenger) {
-    $method = new \ReflectionMethod($legacy_messenger, 'getMessengerService');
-    $method->setAccessible(TRUE);
-    return $method->invoke($legacy_messenger);
+  public function testRemoveSingleMessage() {
+
+    // Set two messages.
+    $this->messenger->addStatus('First message (removed).');
+    $this->messenger->addStatus(t('Second message with <em>markup!</em> (not removed).'));
+    $messages = $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS);
+    // Remove the first.
+    unset($messages[0]);
+
+    // Re-add the second.
+    foreach ($messages as $message) {
+      $this->messenger->addStatus($message);
+    }
+
+    // Check we only have the second one.
+    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
+    $this->assertContains('Second message with <em>markup!</em> (not removed).', $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
+
   }
 
   /**
-   * @covers \Drupal::messenger
-   * @covers ::getMessengerService
+   * Tests we don't add duplicates.
+   *
    * @covers ::all
-   * @covers ::addMessage
+   * @covers ::addStatus
+   * @covers ::addWarning
    * @covers ::addError
+   * @covers ::deleteByType
+   * @covers ::deleteAll
+   */
+  public function testAddNoDuplicates() {
+
+    $this->messenger->addStatus('Non Duplicated status message');
+    $this->messenger->addStatus('Non Duplicated status message');
+
+    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
+
+    $this->messenger->addWarning('Non Duplicated warning message');
+    $this->messenger->addWarning('Non Duplicated warning message');
+
+    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_WARNING));
+
+    $this->messenger->addError('Non Duplicated error message');
+    $this->messenger->addError('Non Duplicated error message');
+
+    $messages = $this->messenger->messagesByType(MessengerInterface::TYPE_ERROR);
+    $this->assertCount(1, $messages);
+
+    // Check getting all messages.
+    $messages = $this->messenger->all();
+    $this->assertCount(3, $messages);
+    $this->assertArrayHasKey(MessengerInterface::TYPE_STATUS, $messages);
+    $this->assertArrayHasKey(MessengerInterface::TYPE_WARNING, $messages);
+    $this->assertArrayHasKey(MessengerInterface::TYPE_ERROR, $messages);
+
+    // Check deletion.
+    $this->messenger->deleteAll();
+    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
+    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_WARNING));
+    $this->assertCount(0, $this->messenger->messagesByType(MessengerInterface::TYPE_ERROR));
+
+  }
+
+  /**
+   * Tests we do add duplicates with repeat flag.
+   *
    * @covers ::addStatus
    * @covers ::addWarning
+   * @covers ::addError
+   * @covers ::deleteByType
    */
-  public function testMessages() {
-    // Save the current container for later use.
-    $container = \Drupal::getContainer();
-
-    // Unset the container to mimic not having one.
-    \Drupal::unsetContainer();
-
-    /** @var \Drupal\Core\Messenger\LegacyMessenger $messenger */
-    // Verify that the Messenger service doesn't exists.
-    $messenger = \Drupal::messenger();
-    $this->assertNull($this->getMessengerService($messenger));
-
-    // Add messages.
-    $messenger->addMessage('Foobar');
-    $messenger->addError('Foo');
-
-    // Verify that retrieving another instance and adding more messages works.
-    $messenger = \Drupal::messenger();
-    $messenger->addStatus('Bar');
-    $messenger->addWarning('Fiz');
-
-    // Restore the container.
-    \Drupal::setContainer($container);
-
-    // Verify that the Messenger service exists.
-    $messenger = \Drupal::messenger();
-    $this->assertInstanceOf(Messenger::class, $this->getMessengerService($messenger));
-
-    // Add more messages.
-    $messenger->addMessage('Platypus');
-    $messenger->addError('Rhinoceros');
-    $messenger->addStatus('Giraffe');
-    $messenger->addWarning('Cheetah');
-
-    // Verify that all the messages are present and accounted for.
-    $messages = $messenger->all();
-    $this->assertContains('Foobar', $messages[MessengerInterface::TYPE_STATUS]);
-    $this->assertContains('Foo', $messages[MessengerInterface::TYPE_ERROR]);
-    $this->assertContains('Bar', $messages[MessengerInterface::TYPE_STATUS]);
-    $this->assertContains('Fiz', $messages[MessengerInterface::TYPE_WARNING]);
-    $this->assertContains('Platypus', $messages[MessengerInterface::TYPE_STATUS]);
-    $this->assertContains('Rhinoceros', $messages[MessengerInterface::TYPE_ERROR]);
-    $this->assertContains('Giraffe', $messages[MessengerInterface::TYPE_STATUS]);
-    $this->assertContains('Cheetah', $messages[MessengerInterface::TYPE_WARNING]);
+  public function testAddWithDuplicates() {
+
+    $this->messenger->addStatus('Duplicated status message', TRUE);
+    $this->messenger->addStatus('Duplicated status message', TRUE);
+
+    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
+
+    $this->messenger->addWarning('Duplicated warning message', TRUE);
+    $this->messenger->addWarning('Duplicated warning message', TRUE);
+
+    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_WARNING));
+
+    $this->messenger->addError('Duplicated error message', TRUE);
+    $this->messenger->addError('Duplicated error message', TRUE);
+
+    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_ERROR));
+
+  }
+
+  /**
+   * Test adding markup.
+   *
+   * @covers ::addStatus
+   * @covers ::deleteByType
+   * @covers ::messagesByType
+   */
+  public function testAddMarkup() {
+
+    // Add a Markup message.
+    $this->messenger->addStatus(Markup::create('Markup with <em>markup!</em>'));
+    // Test duplicate Markup messages.
+    $this->messenger->addStatus(Markup::create('Markup with <em>markup!</em>'));
+
+    $this->assertCount(1, $this->messenger->messagesByType(MessengerInterface::TYPE_STATUS));
+
+    // Ensure that multiple Markup messages work.
+    $this->messenger->addStatus(Markup::create('Markup2 with <em>markup!</em>'));
+
+    $this->assertCount(2, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
+
+    // Test mixing of types.
+    $this->messenger->addStatus(Markup::create('Non duplicate Markup / string.'));
+    $this->messenger->addStatus('Non duplicate Markup / string.');
+    $this->messenger->addStatus(Markup::create('Duplicate Markup / string.'), TRUE);
+    $this->messenger->addStatus('Duplicate Markup / string.', TRUE);
+
+    $this->assertCount(3, $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS));
+
+    $this->messenger->deleteAll();
+
+    // Check translatable string is converted to Markup.
+    $this->messenger->addStatus(new TranslatableMarkup('Translatable message'));
+    $messages = $this->messenger->deleteByType(MessengerInterface::TYPE_STATUS);
+
+    $this->assertInstanceOf(Markup::class, $messages[0]);
+
   }
 
 }