diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 1d05b4a..ecb4c1d 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -514,10 +514,10 @@ function watchdog_exception($type, Exception $exception, $message = NULL, $varia
  * drupal_set_message(t('An error occurred and processing did not complete.'), 'error');
  * @endcode
  *
- * @param string $message
+ * @param string|array|\Drupal\Component\Utility\SafeStringInterface $message
  *   (optional) The translated message to be displayed to the user. For
  *   consistency with other messages, it should begin with a capital letter and
- *   end with a period.
+ *   end with a period. A render array may also be provided for markup.
  * @param string $type
  *   (optional) The message's type. Defaults to 'status'. These values are
  *   supported:
@@ -526,7 +526,10 @@ function watchdog_exception($type, Exception $exception, $message = NULL, $varia
  *   - 'error'
  * @param bool $repeat
  *   (optional) If this is FALSE and the message is already set, then the
- *   message won't be repeated. Defaults to FALSE.
+ *   message won't be repeated. Defaults to FALSE. SafeStrings and strings which
+ *   are equivalent will not be repeated as the in_array() comparison is not
+ *   strict. However, if a string is passed once as a render array and once as
+ *   string or SafeString this will repeated regardless of this setting.
  *
  * @return array|null
  *   A multidimensional array with keys corresponding to the set message types.
@@ -562,9 +565,13 @@ function drupal_set_message($message = NULL, $type = 'status', $repeat = FALSE)
     }
 
     $new = array(
-      'safe' => SafeMarkup::isSafe($message),
-      'message' => (string) $message,
+      // Only preserve safeness for strings. Objects and render arrays do not
+      // need to be marked as safe.
+      'safe' => (is_string($message) && SafeMarkup::isSafe($message)),
+      'message' => $message,
     );
+    // Do not use strict type checking so that equivalent string and
+    // SafeStringInterface objects are detected.
     if ($repeat || !in_array($new, $_SESSION['messages'][$type])) {
       $_SESSION['messages'][$type][] = $new;
     }
@@ -610,6 +617,8 @@ function drupal_get_messages($type = NULL, $clear_queue = TRUE) {
         // the messages also needs to be stored in the session. We retrieve the
         // safe status here and determine whether to mark the string as safe or
         // let autoescape do its thing. See drupal_set_message().
+        // It's only nessasary to set the message as 'safe' when the message is
+        // not a renderable array.
         if ($message['safe']) {
           $message['message'] = SafeMarkup::set($message['message']);
         }
diff --git a/core/modules/system/src/Tests/Bootstrap/DrupalSetMessageTest.php b/core/modules/system/src/Tests/Bootstrap/DrupalSetMessageTest.php
index 37a1663..4c4ce60 100644
--- a/core/modules/system/src/Tests/Bootstrap/DrupalSetMessageTest.php
+++ b/core/modules/system/src/Tests/Bootstrap/DrupalSetMessageTest.php
@@ -24,23 +24,40 @@ class DrupalSetMessageTest extends WebTestBase {
   public static $modules = array('system_test');
 
   /**
-   * Tests setting messages and removing one before it is displayed.
+   * Tests drupal_set_message().
    */
-  function testSetRemoveMessages() {
+  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->assertText('Second message (not removed).');
-  }
+    $this->assertRaw(t('Second message with <em>markup!</em> (not removed).'));
 
-  /**
-   * Tests setting duplicated messages.
-   */
-  function testDuplicatedMessages() {
-    $this->drupalGet('system-test/drupal-set-message');
+    // Ensure duplicate messages are handled as expected.
     $this->assertUniqueText('Non Duplicated message');
     $this->assertNoUniqueText('Duplicated message');
+
+    // Ensure render arrays are rendered.
+    $this->assertRaw('Render array with <em>markup!</em>');
+    $this->assertUniqueText('Render array with markup!');
+    $this->assertRaw('Render array 2 with <em>markup!</em>');
+
+    // Ensure render arrays with objects are rendered.
+    $this->assertRaw('Render array with <em>SafeString markup!</em>');
+    $this->assertUniqueText('Render array with SafeString markup!');
+
+    // Ensure SafeString objects are rendered as expected.
+    $this->assertRaw('SafeString with <em>markup!</em>');
+    $this->assertUniqueText('SafeString with markup!');
+    $this->assertRaw('SafeString2 with <em>markup!</em>');
+
+    // Ensure when the same message is of different types it is not duplicated.
+    $this->assertUniqueText('Non duplicate SafeString / string.');
+    $this->assertNoUniqueText('Duplicate SafeString / string.');
+
+    // Render arrays and strings which are equivalent can not be de-duplicated.
+    // Test this known shortcoming.
+    $this->assertNoUniqueText('Duplicate render array / string');
   }
 
 }
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 5ac7ca0..537887f 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
@@ -10,6 +10,7 @@
 use Drupal\Core\Access\AccessResult;
 use Drupal\Core\Controller\ControllerBase;
 use Drupal\Core\Render\RendererInterface;
+use Drupal\Core\Render\SafeString;
 use Drupal\Core\Session\AccountInterface;
 use Drupal\Core\Url;
 use Symfony\Component\HttpFoundation\RedirectResponse;
@@ -101,7 +102,7 @@ public function mainContentFallback() {
   public function drupalSetMessageTest() {
     // Set two messages.
     drupal_set_message('First message (removed).');
-    drupal_set_message('Second message (not removed).');
+    drupal_set_message(t('Second message with <em>markup!</em> (not removed).'));
 
     // Remove the first.
     unset($_SESSION['messages']['status'][0]);
@@ -112,6 +113,38 @@ public function drupalSetMessageTest() {
 
     drupal_set_message('Duplicated message', 'status', TRUE);
     drupal_set_message('Duplicated message', 'status', TRUE);
+
+    // Add a render array message.
+    drupal_set_message(['#markup' => 'Render array with <em>markup!</em>']);
+    // Test duplicate render arrays.
+    drupal_set_message(['#markup' => 'Render array with <em>markup!</em>']);
+    // Ensure that multiple render array messages work.
+    drupal_set_message(['#markup' => 'Render array 2 with <em>markup!</em>']);
+
+    // Add a render array message containing objects.
+    drupal_set_message(['#markup' => SafeString::create('Render array with <em>SafeString markup!</em>')]);
+    // Test duplicate render array messages.
+    drupal_set_message(['#markup' => SafeString::create('Render array with <em>SafeString markup!</em>')]);
+
+    // Add a SafeString message.
+    drupal_set_message(SafeString::create('SafeString with <em>markup!</em>'));
+    // Test duplicate SafeString messages.
+    drupal_set_message(SafeString::create('SafeString with <em>markup!</em>'));
+    // Ensure that multiple SafeString messages work.
+    drupal_set_message(SafeString::create('SafeString2 with <em>markup!</em>'));
+
+    // Test mixing of types.
+    drupal_set_message(SafeString::create('Non duplicate SafeString / string.'));
+    drupal_set_message('Non duplicate SafeString / string.');
+
+    drupal_set_message(SafeString::create('Duplicate SafeString / string.'), 'status', TRUE);
+    drupal_set_message('Duplicate SafeString / string.', 'status', TRUE);
+
+    // This mixing of types will cause a duplicate even though it is not
+    // intended.
+    drupal_set_message(['#markup' => 'Duplicate render array / string']);
+    drupal_set_message('Duplicate render array / string');
+
     return [];
   }