diff --git a/core/lib/Drupal/Core/Messenger/BaseMessenger.php b/core/lib/Drupal/Core/Messenger/BaseMessenger.php
index c71b587154..f76c295301 100644
--- a/core/lib/Drupal/Core/Messenger/BaseMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/BaseMessenger.php
@@ -12,27 +12,6 @@
  */
 abstract class BaseMessenger implements MessengerInterface {
 
-  /**
-   * {@inheritdoc}
-   */
-  public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
-    $message = $this->convertToMarkup($message);
-    if ($this->shouldAddMessage($message, $type, $repeat)) {
-      $this->doAddMessage($message, $type);
-    }
-    return $this;
-  }
-
-  /**
-   * Add a message in an implementation-specific way.
-   *
-   * @param string|\Drupal\Component\Render\MarkupInterface $message
-   *   The message.
-   * @param string $type
-   *   The message type.
-   */
-  abstract protected function doAddMessage($message, $type);
-
   /**
    * {@inheritdoc}
    */
@@ -70,23 +49,4 @@ protected function convertToMarkup($message) {
     return $message;
   }
 
-  /**
-   * Returns whether a message should be added or not.
-   *
-   * @param string|\Drupal\Component\Render\MarkupInterface $message
-   *   The message.
-   * @param string $type
-   *   The type.
-   * @param bool $repeat
-   *   Whether to repeat or not.
-   *
-   * @return bool
-   *   TRUE if the message should be added, FALSE otherwise.
-   */
-  protected function shouldAddMessage($message, $type, $repeat) {
-    // Do not use strict type checking so that equivalent string and
-    // MarkupInterface objects are detected.
-    return $repeat || !in_array($message, $this->messagesByType($type));
-  }
-
 }
diff --git a/core/lib/Drupal/Core/Messenger/ChainedMessenger.php b/core/lib/Drupal/Core/Messenger/ChainedMessenger.php
index 1356dfde26..b056a2ba7d 100644
--- a/core/lib/Drupal/Core/Messenger/ChainedMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/ChainedMessenger.php
@@ -123,7 +123,7 @@ protected function getMessenger() {
    * @param \Drupal\Core\Messenger\MessengerInterface $to
    *   The messenger to merge to.
    */
-  protected function mergeMessages(MessengerInterface $from, MessengerInterface $to) {
+  public function mergeMessages(MessengerInterface $from, MessengerInterface $to) {
     foreach ($from->all() as $type => $messages) {
       foreach ($messages as $message) {
         $to->addMessage($message, $type);
diff --git a/core/lib/Drupal/Core/Messenger/MemoryMessenger.php b/core/lib/Drupal/Core/Messenger/MemoryMessenger.php
index 056b0e8a3d..494a23a8e5 100644
--- a/core/lib/Drupal/Core/Messenger/MemoryMessenger.php
+++ b/core/lib/Drupal/Core/Messenger/MemoryMessenger.php
@@ -16,13 +16,24 @@ class MemoryMessenger extends BaseMessenger {
    *
    * @var array
    */
-  private $messages = [];
+  protected $messages = [
+    MessengerInterface::TYPE_ERROR => [],
+    MessengerInterface::TYPE_STATUS => [],
+    MessengerInterface::TYPE_WARNING => [],
+  ];
 
   /**
    * {@inheritdoc}
    */
-  protected function doAddMessage($message, $type) {
-    $this->messages[$type][] = $message;
+  public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
+    $message = $this->convertToMarkup($message);
+    // Do not use strict type checking so that equivalent string and
+    // MarkupInterface objects are detected.
+    if ($repeat || !in_array($message, $this->messages[$type])) {
+      $this->messages[$type][] = $message;
+    }
+
+    return $this;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Messenger/Messenger.php b/core/lib/Drupal/Core/Messenger/Messenger.php
index 2859070479..a846defd01 100644
--- a/core/lib/Drupal/Core/Messenger/Messenger.php
+++ b/core/lib/Drupal/Core/Messenger/Messenger.php
@@ -43,7 +43,12 @@ public function __construct(FlashBagInterface $flash_bag, KillSwitch $killSwitch
    * {@inheritdoc}
    */
   public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
-    parent::addMessage($message, $type, $repeat);
+    $message = $this->convertToMarkup($message);
+    // Do not use strict type checking so that equivalent string and
+    // MarkupInterface objects are detected.
+    if ($repeat || !in_array($message, $this->flashBag->peek($type))) {
+      $this->flashBag->add($type, $message);
+    }
 
     // Mark this page as being uncacheable.
     $this->killSwitch->trigger();
@@ -51,13 +56,6 @@ public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE)
     return $this;
   }
 
-  /**
-   * {@inheritdoc}
-   */
-  protected function doAddMessage($message, $type) {
-    $this->flashBag->add($type, $message);
-  }
-
   /**
    * {@inheritdoc}
    */
diff --git a/core/tests/Drupal/Tests/Core/Messenger/ChainedMessengerTest.php b/core/tests/Drupal/Tests/Core/Messenger/ChainedMessengerTest.php
new file mode 100644
index 0000000000..d79e50ae74
--- /dev/null
+++ b/core/tests/Drupal/Tests/Core/Messenger/ChainedMessengerTest.php
@@ -0,0 +1,41 @@
+<?php
+
+namespace Drupal\Tests\Core\Messenger;
+
+use Drupal\Core\Messenger\ChainedMessenger;
+use Drupal\Core\Messenger\MemoryMessenger;
+use Drupal\Core\Messenger\MessengerInterface;
+use Drupal\Tests\UnitTestCase;
+
+/**
+ * @group Messenger
+ * @coversDefaultClass \Drupal\Core\Messenger\ChainedMessenger
+ */
+class ChainedMessengerTest extends UnitTestCase {
+
+  /**
+   * @covers ::mergeMessages
+   */
+  public function testMergeMessages() {
+
+    $messengerFrom = new MemoryMessenger();
+    $messengerFrom->addWarning("Foo");
+    $messengerFrom->addStatus("Bar");
+    $messengerFrom->addError("Fiz");
+
+    $messengerTo = new MemoryMessenger();
+    $messengerTo->addWarning("Platypus");
+
+    $chainedMessenger = new ChainedMessenger();
+    $chainedMessenger->mergeMessages($messengerFrom, $messengerTo);
+
+    $messages = $messengerTo->all();
+
+    $this->assertContains("Platypus", $messages[MessengerInterface::TYPE_WARNING]);
+    $this->assertContains("Foo", $messages[MessengerInterface::TYPE_WARNING]);
+    $this->assertContains("Bar", $messages[MessengerInterface::TYPE_STATUS]);
+    $this->assertContains("Fiz", $messages[MessengerInterface::TYPE_ERROR]);
+
+  }
+
+}