diff --git a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
index 96e9017..0084f67 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/DrupalUnitTestBase.php
@@ -115,7 +115,7 @@ protected function setUp() {
       }
       $class = get_parent_class($class);
     }
-    $this->enableModules($modules, FALSE);
+    $this->enableModules(array_unique($modules), FALSE);
   }
 
   /**
diff --git a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
index 5ac4017..f1c6376 100644
--- a/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
+++ b/core/modules/simpletest/lib/Drupal/simpletest/WebTestBase.php
@@ -829,6 +829,7 @@ protected function setUp() {
       $class = get_parent_class($class);
     }
     if ($modules) {
+      $modules = array_unique($modules);
       $success = module_enable($modules, TRUE);
       $this->assertTrue($success, t('Enabled modules: %modules', array('%modules' => implode(', ', $modules))));
       $this->rebuildContainer();