diff --git a/core/modules/update/update.module b/core/modules/update/update.module
index 9eba38f..790e2b1 100644
--- a/core/modules/update/update.module
+++ b/core/modules/update/update.module
@@ -473,10 +473,10 @@ function update_fetch_data_finished($success, $results) {
 function update_mail($key, &$message, $params) {
   $langcode = $message['langcode'];
   $language = language_load($langcode);
-  $message['subject'] .= t('Site notice for !site_name', array('!site_name' => \Drupal::config('system.site')->get('name')), array('langcode' => $langcode));
+  $message['subject'] = t('Site notice for !site_name', array('!site_name' => \Drupal::config('system.site')->get('name')), array('langcode' => $langcode));
   foreach ($params as $msg_type => $msg_reason) {
     if (in_array($msg_reason, array(UPDATE_NOT_SECURE, UPDATE_REVOKED))) {
-      $message['subject'] .= t('Security notice for !site_name', array('!site_name' => \Drupal::config('system.site')->get('name')), array('langcode' => $langcode));
+      $message['subject'] = t('Security notice for !site_name', array('!site_name' => \Drupal::config('system.site')->get('name')), array('langcode' => $langcode));
     }
     $message['body'][] = _update_message_text($msg_type, $msg_reason, FALSE, $langcode);
   }