diff --git a/core/lib/Drupal/Core/Messenger/Messenger.php b/core/lib/Drupal/Core/Messenger/Messenger.php
index e7aab7724c..e0707304ad 100644
--- a/core/lib/Drupal/Core/Messenger/Messenger.php
+++ b/core/lib/Drupal/Core/Messenger/Messenger.php
@@ -2,6 +2,8 @@
 
 namespace Drupal\Core\Messenger;
 
+use Drupal\Component\Render\MarkupInterface;
+use Drupal\Core\Render\Markup;
 use Symfony\Component\HttpFoundation\Session\Flash\FlashBagInterface;
 
 /**
@@ -32,7 +34,16 @@ public function __construct(FlashBagInterface $flash_bag) {
    * {@inheritdoc}
    */
   public function addMessage($message, $type = self::TYPE_STATUS, $repeat = FALSE) {
-    $this->flashBag->add($type, $message);
+    // Convert strings which are safe to the simplest Markup objects.
+    if (!($message instanceof Markup) && $message instanceof MarkupInterface) {
+      $message = Markup::create((string) $message);
+    }
+    // Do not use strict type checking so that equivalent string and
+    // MarkupInterface objects are detected.
+    if ($repeat || !in_array($message, $this->flashBag->peek($type))) {
+      $this->flashBag->add($type, $message);
+    }
+    return $this;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Messenger/MessengerInterface.php b/core/lib/Drupal/Core/Messenger/MessengerInterface.php
index 216835bcea..217da76575 100644
--- a/core/lib/Drupal/Core/Messenger/MessengerInterface.php
+++ b/core/lib/Drupal/Core/Messenger/MessengerInterface.php
@@ -109,11 +109,15 @@ public function all();
    *   or self::TYPE_ERROR.
    *
    * @return string[]|\Drupal\Component\Render\MarkupInterface[]
+   *   The messages of given type.
    */
   public function messagesByType($type);
 
   /**
    * Deletes all messages.
+   *
+   * @return string[]|\Drupal\Component\Render\MarkupInterface[]
+   *    The deleted messages.
    */
   public function deleteAll();
 
@@ -123,6 +127,9 @@ public function deleteAll();
    * @param string $type
    *   The messages' type. Either self::TYPE_STATUS, self::TYPE_WARNING, or
    *   self::TYPE_ERROR.
+   *
+   * @return string[]|\Drupal\Component\Render\MarkupInterface[]
+   *   The deleted messages of given type..
    */
   public function deleteByType($type);
 
diff --git a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
index 5e49c2403b..6ed439e632 100644
--- a/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
+++ b/core/modules/system/tests/modules/system_test/src/Controller/SystemTestController.php
@@ -96,13 +96,6 @@ public function mainContentFallback() {
    *   Empty string, we just test the setting of messages.
    */
   public function drupalSetMessageTest() {
-    // Set two messages.
-    drupal_set_message('First message (removed).');
-    drupal_set_message(t('Second message with <em>markup!</em> (not removed).'));
-
-    // Remove the first.
-    unset($_SESSION['messages']['status'][0]);
-
     // Duplicate message check.
     drupal_set_message('Non Duplicated message', 'status', FALSE);
     drupal_set_message('Non Duplicated message', 'status', FALSE);
diff --git a/core/modules/system/tests/src/Functional/Bootstrap/DrupalSetMessageTest.php b/core/modules/system/tests/src/Functional/Bootstrap/DrupalSetMessageTest.php
index 976e8bebfe..ff207b5f3f 100644
--- a/core/modules/system/tests/src/Functional/Bootstrap/DrupalSetMessageTest.php
+++ b/core/modules/system/tests/src/Functional/Bootstrap/DrupalSetMessageTest.php
@@ -22,11 +22,7 @@ class DrupalSetMessageTest extends BrowserTestBase {
    * Tests drupal_set_message().
    */
   public function testDrupalSetMessage() {
-    // The page at system-test/drupal-set-message sets two messages and then
-    // removes the first before it is displayed.
     $this->drupalGet('system-test/drupal-set-message');
-    $this->assertNoText('First message (removed).');
-    $this->assertRaw(t('Second message with <em>markup!</em> (not removed).'));
 
     // Ensure duplicate messages are handled as expected.
     $this->assertUniqueText('Non Duplicated message');
diff --git a/core/tests/Drupal/KernelTests/Core/Common/DrupalSetMessageTest.php b/core/tests/Drupal/KernelTests/Core/Common/DrupalSetMessageTest.php
index 59470e6563..7a15fc3e04 100644
--- a/core/tests/Drupal/KernelTests/Core/Common/DrupalSetMessageTest.php
+++ b/core/tests/Drupal/KernelTests/Core/Common/DrupalSetMessageTest.php
@@ -20,10 +20,4 @@ public function testDrupalSetMessage() {
     $this->assertEquals('A message: bar', (string) $messages['status'][0]);
   }
 
-  protected function tearDown() {
-    // Clear session to prevent global leakage.
-    unset($_SESSION['messages']);
-    parent::tearDown();
-  }
-
 }