diff --git a/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
index 3a26493..65d2afe 100644
--- a/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -580,6 +580,43 @@ protected function processExtension(array &$cache, $name, $type, $theme, $path)
   }
 
   /**
+   * Completes the definition of the requested suggestion hook.
+   *
+   * @param string $hook
+   *   The name of the suggestion hook to complete.
+   * @param array $cache
+   *   The theme registry, as documented in
+   *   \Drupal\Core\Theme\Registry::processExtension().
+   */
+  protected function completeSuggestion($hook, array &$cache) {
+    $previous_hook = $hook;
+    $incomplete_previous_hook = array();
+    while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions']))
+      && $pos = strrpos($previous_hook, '__')) {
+      if (isset($cache[$previous_hook]) && !$incomplete_previous_hook && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
+        $incomplete_previous_hook = $cache[$previous_hook];
+        unset($incomplete_previous_hook['incomplete preprocess functions']);
+      }
+      $previous_hook = substr($previous_hook, 0, $pos);
+
+      // If base hook exists clone of it for the preprocess function
+      // without a template.
+      // @see https://www.drupal.org/node/2457295
+      if (isset($cache[$previous_hook]) && !isset($cache[$previous_hook]['incomplete preprocess functions'])) {
+        $cache[$hook] = $incomplete_previous_hook + $cache[$previous_hook];
+        if (isset($incomplete_previous_hook['preprocess functions'])) {
+          $diff = array_diff($incomplete_previous_hook['preprocess functions'], $cache[$previous_hook]['preprocess functions']);
+          $cache[$hook]['preprocess functions'] = array_merge($cache[$previous_hook]['preprocess functions'], $diff);
+        }
+        // If a base hook isn't set, this is the actual base hook.
+        if (!isset($cache[$previous_hook]['base hook'])) {
+          $cache[$hook]['base hook'] = $previous_hook;
+        }
+      }
+    }
+  }
+
+  /**
    * Merges the preceding hook's preprocess functions into the current hook's.
    *
    * @param string $hook
@@ -613,43 +650,6 @@ protected function mergePreprocessFunctions($hook, $complete_hook, array &$cache
   }
 
   /**
-   * Completes the definition of the requested suggestion hook.
-   *
-   * @param string $hook
-   *   The name of the suggestion hook to complete.
-   * @param array $cache
-   *   The theme registry, as documented in
-   *   \Drupal\Core\Theme\Registry::processExtension().
-   */
-  protected function completeSuggestion($hook, array &$cache) {
-    $previous_hook = $hook;
-    // Loop until we find a preceding completed hook definition.
-    while ((!isset($cache[$previous_hook]) || isset($cache[$previous_hook]['incomplete preprocess functions']))
-      && $pos = strrpos($previous_hook, '__')) {
-      $previous_hook = substr($previous_hook, 0, $pos);
-
-      if (!isset($cache[$previous_hook])) {
-        continue;
-      }
-
-      // If there is a incomplete hook and we are creating a new theme registry
-      // entry, copy it as a base for this one.
-      if (!isset($cache[$hook]) && isset($cache[$previous_hook]['incomplete preprocess functions'])) {
-        $cache[$hook] = $cache[$previous_hook];
-      }
-      // First found completed hook has the correct set of preprocess functions.
-      if (!isset($cache[$previous_hook]['incomplete preprocess functions'])) {
-        $this->mergePreprocessFunctions($hook, $previous_hook, $cache);
-
-        // If a base hook isn't set, this is the actual base hook.
-        if (!isset($cache[$previous_hook]['base hook'])) {
-          $cache[$hook]['base hook'] = $previous_hook;
-        }
-      }
-    }
-  }
-
-  /**
    * Completes the theme registry adding discovered functions and hooks.
    *
    * @param array $cache
@@ -707,7 +707,11 @@ protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
     ksort($suggestion_level);
     foreach ($suggestion_level as $level => $item) {
       foreach ($item as $preprocessor => $hook) {
-        if (strpos($hook, '__')) {
+        if (isset($cache[$hook]['preprocess functions']) && !in_array($hook, $cache[$hook]['preprocess functions'])) {
+          // Add missing preprocessor to existing hook.
+          $cache[$hook]['preprocess functions'][] = $preprocessor;
+        }
+        elseif (!isset($cache[$hook]) && strpos($hook, '__')) {
           // Process non-existing hook and register it.
           // Look for a previously defined hook that is either a less specific
           // suggestion hook or the base hook.
diff --git a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
index a5f9a47..f3eb364 100644
--- a/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
+++ b/core/tests/Drupal/Tests/Core/Theme/RegistryTest.php
@@ -199,29 +199,6 @@ public function setUpPostProcessExtension() {
     }
 
     /**
-     * Tests that suggestion preprocess functions are added to the registry.
-     *
-     * @param array $cache
-     *   The incomplete theme registry.
-     * @param array $functions
-     *   The preprocess functions that are defined.
-     * @param array $expected
-     *   The completed theme registry.
-     *
-     * @dataProvider providerPreprocessFunctionCompletion
-     */
-    public function testPreprocessFunctionCompletion(array $cache, array $functions, array $expected) {
-
-      $this->setUpPostProcessExtension();
-      self::$functions['user'] = $functions;
-
-      $this->reflectionMethod->invokeArgs($this->registry, [ & $cache, $this->theme]);
-
-      $this->assertArrayEquals($cache, $expected);
-
-    }
-
-    /**
      * Tests that preprocess functions are added when base hook is specified.
      */
     public function testManualBaseHook() {
@@ -229,30 +206,28 @@ public function testManualBaseHook() {
       $functions = ['cat', 'mouse', 'cheese'];
       self::$functions['user'] = $functions;
       // Define a test case registry with mock data.
-      $cache = [
-      'test_hook' =>
-      ['preprocess functions' => ['cat', 'mouse'], ],
-      'test_hook_manual' =>
-      ['preprocess functions' => ['cheese'],
-      'base hook' => 'test_hook',
-      'incomplete preprocess functions' => TRUE, ]
+      $hooks = [
+        'test_hook' =>
+        ['preprocess functions' => ['cat', 'mouse'], ],
+        'test_hook_manual' =>
+        ['preprocess functions' => ['cheese'],
+          'base hook' => 'test_hook',
+          'incomplete preprocess functions' => TRUE, ]
       ];
 
       $expected = [
-      'test_hook' =>
-      ['preprocess functions' => ['cat', 'mouse'],
-      ],
-      'test_hook_manual' =>
-      ['preprocess functions' => ['cat', 'mouse', 'cheese'],
-      'base hook' => 'test_hook',
-      ]
+        'test_hook' =>
+        ['preprocess functions' => ['cat', 'mouse'], ],
+        'test_hook_manual' =>
+        ['preprocess functions' => ['cat', 'mouse', 'cheese'],
+          'base hook' => 'test_hook', ]
       ];
 
       $this->setUpPostProcessExtension();
 
-      $this->reflectionMethod->invokeArgs($this->registry, [ & $cache, $this->theme]);
+      $this->reflectionMethod->invokeArgs($this->registry, [ &$hooks, $this->theme]);
 
-      $this->assertArrayEquals($cache, $expected);
+      $this->assertArrayEquals($hooks, $expected);
 
     }
 
@@ -265,36 +240,35 @@ public function testManualSuggestion() {
 
       $functions = ['cat', 'mouse', 'cheese'];
 
-      $cache = [
-      'test_hook' =>
-      ['preprocess functions' => ['cat', 'mouse'], ],
-      'test_hook__suggestion' =>
-      ['preprocess functions' => ['bread'],
-      'incomplete preprocess functions' => TRUE, ],
-      'test_hook__suggestion__another' =>
-      ['preprocess functions' => ['cheese'],
-      'incomplete preprocess functions' => TRUE, ]
-
+      $hooks = [
+        'test_hook' =>
+        ['preprocess functions' => ['cat', 'mouse'],
+          'template' => 'test-hook'],
+        'test_hook__suggestion' =>
+        ['preprocess functions' => ['bread'],
+          'incomplete preprocess functions' => TRUE, ],
+        'test_hook__suggestion__another' =>
+        ['preprocess functions' => ['cheese'],
+          'incomplete preprocess functions' => TRUE, ]
       ];
 
       $expected = [
-      'test_hook' =>
-      ['preprocess functions' => ['cat', 'mouse'],
-      ],
-      'test_hook__suggestion' =>
-      ['preprocess functions' => ['cat', 'mouse', 'bread',
-                               'test_preprocess_test_hook__suggestion'],
-      'base hook' => 'test_hook',
-      ],
-      'test_hook__suggestion__another' =>
-      ['preprocess functions' =>
-        ['cat', 'mouse', 'bread',
-          'test_preprocess_test_hook__suggestion',
-          'cheese',
-          'test_preprocess_test_hook__suggestion__another'],
-      'base hook' => 'test_hook',
-      ]
-
+        'test_hook' =>
+        ['preprocess functions' => ['cat', 'mouse'],
+          'template' => 'test-hook', ],
+        'test_hook__suggestion' =>
+        ['preprocess functions' => ['cat', 'mouse', 'bread',
+                                 'test_preprocess_test_hook__suggestion'],
+          'base hook' => 'test_hook',
+          'template' => 'test-hook', ],
+        'test_hook__suggestion__another' =>
+        ['preprocess functions' =>
+          ['cat', 'mouse', 'bread',
+            'test_preprocess_test_hook__suggestion',
+            'cheese',
+            'test_preprocess_test_hook__suggestion__another'],
+          'base hook' => 'test_hook',
+          'template' => 'test-hook', ]
       ];
 
       $functions = ['cat', 'mouse', 'cheese', 'bread',
@@ -302,266 +276,9 @@ public function testManualSuggestion() {
                    'test_preprocess_test_hook__suggestion__another'];
       self::$functions['user'] = $functions;
 
-      $this->reflectionMethod->invokeArgs($this->registry, [ & $cache, $this->theme]);
+      $this->reflectionMethod->invokeArgs($this->registry, [ &$hooks, $this->theme]);
 
-      $this->assertArrayEquals($cache, $expected);
-    }
-
-
-    /**
-     * Provides test data for testPreprocessFunctionCompletion().
-     *
-     * @return array
-     *   Test data.
-     */
-    public function providerPreprocessFunctionCompletion() {
-      // A pairwise test set listing which suggestion hooks should be defined.
-      // We test up to three suggestions. For the third level suggestion hook
-      // unset hook with no preprocess is not included in the test set.
-      //
-      // Each line determines a test case.
-      // First array determines theme hooks that are defined:
-      // - First column: hook__s1 is defined in the registry.
-      // - Second column: hook__s1__s2 is defined in the registry.
-      // - Third column: hook__s1__s2__s3 is defined in the registry.
-      //
-      // Second array determines preprocess functions that are defined:
-      // - First column: test_preprocess_hook__s1 is defined.
-      // - Second column: test_preprocess_hook__s1__s2 is defined.
-      // - Third column: test_preprocess_hook__s1__s2__s3 is defined.
-      $test_definitions = [
-      [[TRUE, TRUE, FALSE], [TRUE, TRUE, TRUE]],
-      [[TRUE, FALSE, TRUE], [FALSE, FALSE, TRUE]],
-      [[FALSE, TRUE, TRUE], [FALSE, FALSE, FALSE]],
-      [[FALSE, FALSE, TRUE], [TRUE, TRUE, TRUE]],
-      [[TRUE, TRUE, TRUE], [FALSE, TRUE, FALSE]],
-      [[FALSE, TRUE, TRUE], [TRUE, FALSE, FALSE]],
-      [[FALSE, FALSE, FALSE], [FALSE, FALSE, TRUE]],
-      [[TRUE, FALSE, TRUE], [FALSE, TRUE, FALSE]],
-      [[FALSE, TRUE, TRUE], [TRUE, FALSE, TRUE]],
-      ];
-
-      $expected_test_outputs = [
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1',
-      'preprocess functions' => [
-        'template_preprocess','test_preprocess_hook__s1']],
-      'hook__s1__s2' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1',
-        'test_preprocess_hook__s1__s2']],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1',
-        'test_preprocess_hook__s1__s2',
-        'test_preprocess_hook__s1__s2__s3'
-      ]]],
-
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1',
-      'preprocess functions' => [
-        'template_preprocess' ]],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2__s3',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1__s2__s3'
-      ]]],
-
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1__s2' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2',
-      'preprocess functions' => [
-        'template_preprocess' ]],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2__s3',
-      'preprocess functions' => [
-        'template_preprocess',
-      ]]],
-
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1' => [
-      'base hook' => 'hook',
-      'template' => 'hook',
-      'preprocess functions' => [
-        'template_preprocess', 'test_preprocess_hook__s1']],
-      'hook__s1__s2' => [
-      'base hook' => 'hook',
-      'template' => 'hook',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1',
-        'test_preprocess_hook__s1__s2']],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2__s3',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1',
-        'test_preprocess_hook__s1__s2',
-        'test_preprocess_hook__s1__s2__s3'
-      ]]],
-
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1',
-      'preprocess functions' => [
-        'template_preprocess']],
-      'hook__s1__s2' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1__s2']],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2__s3',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1__s2',
-      ]]],
-
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1' => [
-      'base hook' => 'hook',
-      'template' => 'hook',
-      'preprocess functions' => [
-        'template_preprocess','test_preprocess_hook__s1']],
-      'hook__s1__s2' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1' ]],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2__s3',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1',
-      ]]],
-
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1__s2__s3'
-      ]]],
-
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1',
-      'preprocess functions' => [
-        'template_preprocess' ]],
-      'hook__s1__s2' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1',
-
-      'preprocess functions' => [
-        'template_preprocess','test_preprocess_hook__s1__s2']],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2__s3',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1__s2'
-      ]]],
-
-      [  'hook' => [
-      'preprocess functions' => ['template_preprocess'],
-      'template' => 'hook'],
-      'hook__s1' => [
-      'base hook' => 'hook',
-      'template' => 'hook',
-      'preprocess functions' => [
-        'template_preprocess','test_preprocess_hook__s1']],
-      'hook__s1__s2' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2',
-      'preprocess functions' => [
-        'template_preprocess','test_preprocess_hook__s1']],
-      'hook__s1__s2__s3' => [
-      'base hook' => 'hook',
-      'template' => 'hook__s1__s2__s3',
-      'preprocess functions' => [
-        'template_preprocess',
-        'test_preprocess_hook__s1',
-        'test_preprocess_hook__s1__s2__s3'
-      ]]],
-
-      ];
-
-      // The theme hook and preprocess function names used in the tests.
-      $hook_names = ['hook__s1', 'hook__s1__s2', 'hook__s1__s2__s3'];
-      $preprocess_function_names = ['test_preprocess_hook__s1',
-                               'test_preprocess_hook__s1__s2',
-                               'test_preprocess_hook__s1__s2__s3'];
-
-      $base_hook = 'hook';
-
-      foreach ($test_definitions as $test_index => $test_definition) {
-
-        $test = [
-        'tested theme hooks' => [ $base_hook  =>
-                            ['preprocess functions' => ['template_preprocess'],
-                              'template' => $base_hook]],
-        'tested preprocess functions' => [],
-        'expected test result' => []
-        ];
-
-        // Add the wanted suggestion theme hooks to test data.
-        foreach ($test_definition[0] as $key => $hook_enabled) {
-          if ($hook_enabled) {
-            $test['tested theme hooks'][$hook_names[$key]]
-              = ['base hook' => $base_hook,
-                'template' => $hook_names[$key],
-                'incomplete preprocess functions' => TRUE];
-          }
-        }
-        // Add the wanted suggestion preprocess functions to test data.
-        foreach ($test_definition[1] as $key => $function_enabled) {
-          if ($function_enabled) {
-            $test['tested preprocess functions'][] = $preprocess_function_names[$key];
-          }
-        }
-        $test['expected test result'] = $expected_test_outputs[$test_index];
-        $tests[] = $test;
-      }
-
-      return $tests;
+      $this->assertArrayEquals($hooks, $expected);
     }
 
     protected function setupTheme($theme_name = NULL) {
@@ -592,7 +309,6 @@ protected function listThemes() {
 
     protected function initializeTheme() {
     }
-
   }
 
 }