diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php b/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
index b830b1a..7e095bb 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Form/SettingsForm.php
@@ -245,7 +245,7 @@ public function getConfigNames() {
     $config_names = array('aggregator.settings');
 
     foreach ($this->getConfigurableInstances() as $instance) {
-      $config_names += $instance->getConfigNames();
+      $config_names = array_merge($config_names, $instance->getConfigNames());
     }
 
     return array_unique($config_names);
diff --git a/core/modules/aggregator/tests/Drupal/aggregator/Tests/Form/SettingsFormTest.php b/core/modules/aggregator/tests/Drupal/aggregator/Tests/Form/SettingsFormTest.php
index 68c7d73..79367dc 100644
--- a/core/modules/aggregator/tests/Drupal/aggregator/Tests/Form/SettingsFormTest.php
+++ b/core/modules/aggregator/tests/Drupal/aggregator/Tests/Form/SettingsFormTest.php
@@ -38,12 +38,12 @@ public function testGetConfigNames() {
     $configs = array('aggregator.settings' => array());
     $definitions = array();
 
-    list($settings_form) = $this->getSettingsForm($configs, $definitions);
+    $settings_form = $this->getSettingsForm($configs, $definitions);
 
     $expected = array('aggregator.settings');
-    $this->assertSame($expected, $settings_form->getConfigNames());
+    $this->assertSame($expected, array_values($settings_form->getConfigNames()));
 
-    // Test that aggregator plugins can add additional configuration names and
+    // Test that aggregator plugins can add additional configuration names.
     // that duplicated names are removed.
     $configs = array('aggregator.settings' => array(
       'fetcher' => 'aggregator_test_fetcher',
@@ -64,28 +64,24 @@ public function testGetConfigNames() {
         'description' => 'Test generic processor functionality.',
       ),
     );
+    $config_names = array('aggregator_test.settings');
 
-    list($settings_form, $plugin_manager) = $this->getSettingsForm($configs, $definitions);
-
-    $plugin = $this->getMock('Drupal\Core\Plugin\PluginFormInterface');
-    $plugin
-      ->expects($this->exactly(3))
-      ->method('getConfigNames')
-      ->will($this->returnValue(
-        array('aggregator.settings', 'aggregator_test.settings')
-      ));
-    $plugin_manager
-      ->expects($this->exactly(3))
-      ->method('createInstance')
-      ->will($this->returnValueMap(array(
-        array('aggregator_test_fetcher', array(), $plugin),
-        array('aggregator_test_parser', array(), $plugin),
-        array('aggregator_test_processor', array(), $plugin)
-      )));
+    $settings_form = $this->getSettingsForm($configs, $definitions, $config_names);
 
     $expected = array('aggregator.settings', 'aggregator_test.settings');
 
-    $this->assertSame($expected, $settings_form->getConfigNames());
+    $this->assertEquals($expected, array_values($settings_form->getConfigNames()));
+
+    // Test that config names are not duplicated.
+    $config_names = array('aggregator.settings');
+
+    // Re-use $configs and $definitions.
+    $settings_form = $this->getSettingsForm($configs, $definitions, $config_names);
+
+    $expected = array('aggregator.settings');
+
+    $this->assertEquals($expected, array_values($settings_form->getConfigNames()));
+
   }
 
   /**
@@ -96,12 +92,13 @@ public function testGetConfigNames() {
    *   UnitTestCase::getConfigFactoryStub().
    * @param array $definitions
    *   An array of aggregator plugin definitions.
+   * @param array $config_names
+   *   An array of config names that the plugins should return.
    *
-   * @return array
-   *   An array that contains the aggregator settings form and mocked aggregator
-   *   plugin manager.
+   * @return \Drupal\aggregator\Form\SettingsForm
+   *   The aggregator settings form.
    */
-  protected function getSettingsForm(array $configs, array $definitions) {
+  protected function getSettingsForm(array $configs, array $definitions, array $config_names = NULL) {
     $plugin_manager = $this->getMockBuilder('Drupal\aggregator\Plugin\AggregatorPluginManager')
       ->disableOriginalConstructor()
       ->getMock();
@@ -109,14 +106,34 @@ protected function getSettingsForm(array $configs, array $definitions) {
       ->expects($this->exactly(3))
       ->method('getDefinitions')
       ->will($this->returnValue($definitions));
-    return array(new SettingsForm(
+
+    $settings_form = new SettingsForm(
       $this->getConfigFactoryStub($configs),
       $this->getMock('Drupal\Core\Config\Context\ContextInterface'),
       $plugin_manager,
       $plugin_manager,
       $plugin_manager,
       $this->getMock('Drupal\Core\StringTranslation\TranslationInterface')
-    ), $plugin_manager);
+    );
+
+    if (!empty($config_names)) {
+      $plugin = $this->getMock('Drupal\Core\Plugin\PluginFormInterface');
+      $plugin
+        ->expects($this->exactly(6))
+        ->method('getConfigNames')
+        ->will($this->returnValue($config_names));
+      $plugin_manager
+        ->expects($this->exactly(3))
+        ->method('createInstance')
+        ->will($this->returnValueMap(array(
+          array('aggregator_test_fetcher', array(), $plugin),
+          array('aggregator_test_parser', array(), $plugin),
+          array('aggregator_test_processor', array(), $plugin)
+        )));
+
+    }
+
+    return $settings_form;
   }
 
 }