diff -u b/core/lib/Drupal/Core/Theme/Registry.php b/core/lib/Drupal/Core/Theme/Registry.php
--- b/core/lib/Drupal/Core/Theme/Registry.php
+++ b/core/lib/Drupal/Core/Theme/Registry.php
@@ -577,12 +577,12 @@
     // Gather prefixes. This will be used to limit the found functions to the
     // expected naming conventions.
     $prefixes = array_keys((array) $this->moduleHandler->getModuleList());
+    foreach (array_reverse($theme->getBaseThemes()) as $base) {
+      $prefixes[] = $base->getName();
+    }
     if ($theme->getEngine()) {
       $prefixes[] = $theme->getEngine() . '_engine';
     }
-    foreach ($theme->getBaseThemes() as $base) {
-      $prefixes[] = $base->getName();
-    }
     $prefixes[] = $theme->getName();
 
     // Collect all known hooks. Discovered functions must be based on a known hook.
@@ -635,6 +635,8 @@
       if (isset($cache[$hook]['preprocess functions']) && empty($cache[$hook]['preprocess functions'])) {
         unset($cache[$hook]['preprocess functions']);
       }
+      // Ensure uniqueness.
+      $cache[$hook]['preprocess functions'] = array_unique($cache[$hook]['preprocess functions']);
     }
   }
 
reverted:
--- b/core/modules/system/src/Tests/Theme/RegistryTest.php
+++ a/core/modules/system/src/Tests/Theme/RegistryTest.php
@@ -66,6 +66,42 @@
   }
 
   /**
+   * Tests the theme registry with multiple subthemes.
+   */
+  public function testMultipleSubThemes() {
+    $theme_handler = \Drupal::service('theme_handler');
+    $theme_handler->install(['test_basetheme', 'test_subtheme', 'test_subsubtheme']);
+
+    $registry_subsub_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_subsubtheme');
+    $registry_subsub_theme->setThemeManager(\Drupal::theme());
+    $registry_sub_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_subtheme');
+    $registry_sub_theme->setThemeManager(\Drupal::theme());
+    $registry_base_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_basetheme');
+    $registry_base_theme->setThemeManager(\Drupal::theme());
+
+    $preprocess_functions = $registry_subsub_theme->get()['theme_test_template_test']['preprocess functions'];
+    $this->assertIdentical([
+      'template_preprocess',
+      'test_basetheme_preprocess_theme_test_template_test',
+      'test_subtheme_preprocess_theme_test_template_test',
+      'test_subsubtheme_preprocess_theme_test_template_test',
+    ], $preprocess_functions);
+
+    $preprocess_functions = $registry_sub_theme->get()['theme_test_template_test']['preprocess functions'];
+    $this->assertIdentical([
+      'template_preprocess',
+      'test_basetheme_preprocess_theme_test_template_test',
+      'test_subtheme_preprocess_theme_test_template_test',
+    ], $preprocess_functions);
+
+    $preprocess_functions = $registry_base_theme->get()['theme_test_template_test']['preprocess functions'];
+    $this->assertIdentical([
+      'template_preprocess',
+      'test_basetheme_preprocess_theme_test_template_test',
+    ], $preprocess_functions);
+  }
+
+  /**
    * Tests that the theme registry can be altered by themes.
    */
   public function testThemeRegistryAlterByTheme() {
reverted:
--- b/core/modules/system/src/Tests/Theme/RegistryTest.php.orig
+++ /dev/null
@@ -1,119 +0,0 @@
-<?php
-
-/**
- * @file
- * Definition of Drupal\system\Tests\Theme\RegistryTest.
- */
-
-namespace Drupal\system\Tests\Theme;
-
-use Drupal\Core\Theme\Registry;
-use Drupal\simpletest\KernelTestBase;
-use Drupal\Core\Utility\ThemeRegistry;
-
-/**
- * Tests the behavior of the ThemeRegistry class.
- *
- * @group Theme
- */
-class RegistryTest extends KernelTestBase {
-
-  /**
-   * Modules to enable.
-   *
-   * @var array
-   */
-  public static $modules = array('theme_test', 'system');
-
-  protected $profile = 'testing';
-
-  /**
-   * Tests the behavior of the theme registry class.
-   */
-  function testRaceCondition() {
-    // The theme registry is not marked as persistable in case we don't have a
-    // proper request.
-    \Drupal::request()->setMethod('GET');
-    $cid = 'test_theme_registry';
-
-    // Directly instantiate the theme registry, this will cause a base cache
-    // entry to be written in __construct().
-    $cache = \Drupal::cache();
-    $lock_backend = \Drupal::lock();
-    $registry = new ThemeRegistry($cid, $cache, $lock_backend, array('theme_registry'), $this->container->get('module_handler')->isLoaded());
-
-    $this->assertTrue(\Drupal::cache()->get($cid), 'Cache entry was created.');
-
-    // Trigger a cache miss for an offset.
-    $this->assertTrue($registry->get('theme_test_template_test'), 'Offset was returned correctly from the theme registry.');
-    // This will cause the ThemeRegistry class to write an updated version of
-    // the cache entry when it is destroyed, usually at the end of the request.
-    // Before that happens, manually delete the cache entry we created earlier
-    // so that the new entry is written from scratch.
-    \Drupal::cache()->delete($cid);
-
-    // Destroy the class so that it triggers a cache write for the offset.
-    $registry->destruct();
-
-    $this->assertTrue(\Drupal::cache()->get($cid), 'Cache entry was created.');
-
-    // Create a new instance of the class. Confirm that both the offset
-    // requested previously, and one that has not yet been requested are both
-    // available.
-    $registry = new ThemeRegistry($cid, $cache, $lock_backend, array('theme_registry'), $this->container->get('module_handler')->isLoaded());
-    $this->assertTrue($registry->get('theme_test_template_test'), 'Offset was returned correctly from the theme registry');
-    $this->assertTrue($registry->get('theme_test_template_test_2'), 'Offset was returned correctly from the theme registry');
-  }
-
-  /**
-   * Tests the theme registry with multiple subthemes.
-   */
-  public function testMultipleSubThemes() {
-    $theme_handler = \Drupal::service('theme_handler');
-    $theme_handler->install(['test_basetheme', 'test_subtheme', 'test_subsubtheme']);
-
-    $registry_subsub_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_subsubtheme');
-    $registry_subsub_theme->setThemeManager(\Drupal::theme());
-    $registry_sub_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_subtheme');
-    $registry_sub_theme->setThemeManager(\Drupal::theme());
-    $registry_base_theme = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_basetheme');
-    $registry_base_theme->setThemeManager(\Drupal::theme());
-
-    $preprocess_functions = $registry_subsub_theme->get()['theme_test_template_test']['preprocess functions'];
-    $this->assertIdentical([
-      'template_preprocess',
-      'test_basetheme_preprocess_theme_test_template_test',
-      'test_subtheme_preprocess_theme_test_template_test',
-      'test_subsubtheme_preprocess_theme_test_template_test',
-    ], $preprocess_functions);
-
-    $preprocess_functions = $registry_sub_theme->get()['theme_test_template_test']['preprocess functions'];
-    $this->assertIdentical([
-      'template_preprocess',
-      'test_basetheme_preprocess_theme_test_template_test',
-      'test_subtheme_preprocess_theme_test_template_test',
-    ], $preprocess_functions);
-
-    $preprocess_functions = $registry_base_theme->get()['theme_test_template_test']['preprocess functions'];
-    $this->assertIdentical([
-      'template_preprocess',
-      'test_basetheme_preprocess_theme_test_template_test',
-    ], $preprocess_functions);
-  }
-
-  /**
-   * Tests that the theme registry can be altered by themes.
-   */
-  public function testThemeRegistryAlterByTheme() {
-
-    /** @var \Drupal\Core\Extension\ThemeHandlerInterface $theme_handler */
-    $theme_handler = \Drupal::service('theme_handler');
-    $theme_handler->install(['test_theme']);
-    $theme_handler->setDefault('test_theme');
-
-    $registry = new Registry(\Drupal::root(), \Drupal::cache(), \Drupal::lock(), \Drupal::moduleHandler(), $theme_handler, \Drupal::service('theme.initialization'), 'test_theme');
-    $registry->setThemeManager(\Drupal::theme());
-    $this->assertEqual('value', $registry->get()['theme_test_template_test']['variables']['additional']);
-  }
-
-}
reverted:
--- b/core/modules/system/tests/themes/test_theme/test_theme.theme.orig
+++ /dev/null
@@ -1,107 +0,0 @@
-<?php
-
-/**
- * @file
- * Theme to help test the Twig engine.
- */
-
-/**
- * Implements THEME_preprocess_twig_theme_test_php_variables().
- */
-function test_theme_preprocess_twig_theme_test_php_variables(&$variables) {
-  $variables['php_values'] = _test_theme_twig_php_values();
-}
-
-/**
- * Tests a theme overriding a suggestion of a base theme hook.
- */
-function test_theme_theme_test__suggestion($variables) {
-  return 'Theme hook implementor=test_theme_theme_test__suggestion(). Foo=' . $variables['foo'];
-}
-
-/**
- * Implements hook_element_info_alter().
- */
-function test_theme_element_info_alter(&$info) {
-  // Decrease the default size of textfields.
-  if (isset($info['textfield']['#size'])) {
-    $info['textfield']['#size'] = 40;
-  }
-}
-
-function test_theme_library_info_alter(&$libraries) {
-  $libraries['kitten']['js'][] = 'kittens.js';
-}
-
-/**
- * Tests a theme implementing an alter hook.
- *
- * The confusing function name here is due to this being an implementation of
- * the alter hook invoked when the 'theme_test' module calls
- * \Drupal::moduleHandler->alter('theme_test_alter').
- */
-function test_theme_theme_test_alter_alter(&$data) {
-  $data = 'test_theme_theme_test_alter_alter was invoked';
-}
-
-/**
- * Implements hook_theme_suggestions_alter().
- */
-function test_theme_theme_suggestions_alter(array &$suggestions, array $variables, $hook) {
-  drupal_set_message(__FUNCTION__ . '() executed.');
-  // Theme alter hooks run after module alter hooks, so add this theme
-  // suggestion to the beginning of the array so that the suggestion added by
-  // the theme_suggestions_test module can be picked up when that module is
-  // enabled.
-  if ($hook == 'theme_test_general_suggestions') {
-    array_unshift($suggestions, 'theme_test_general_suggestions__' . 'theme_override');
-  }
-}
-
-/**
- * Implements hook_theme_suggestions_HOOK_alter().
- */
-function test_theme_theme_suggestions_theme_test_suggestions_alter(array &$suggestions, array $variables) {
-  drupal_set_message(__FUNCTION__ . '() executed.');
-  // Theme alter hooks run after module alter hooks, so add this theme
-  // suggestion to the beginning of the array so that the suggestion added by
-  // the theme_suggestions_test module can be picked up when that module is
-  // enabled.
-  array_unshift($suggestions, 'theme_test_suggestions__' . 'theme_override');
-}
-
-/**
- * Implements hook_theme_suggestions_HOOK_alter().
- */
-function test_theme_theme_suggestions_theme_test_function_suggestions_alter(array &$suggestions, array $variables) {
-  // Theme alter hooks run after module alter hooks, so add this theme
-  // suggestion to the beginning of the array so that the suggestion added by
-  // the theme_suggestions_test module can be picked up when that module is
-  // enabled.
-  array_unshift($suggestions, 'theme_test_function_suggestions__' . 'theme_override');
-}
-
-/**
- * Returns HTML for a theme function suggestion test.
- *
- * Implements the theme_test_function_suggestions__theme_override suggestion.
- */
-function test_theme_theme_test_function_suggestions__theme_override($variables) {
-  return 'Theme function overridden based on new theme suggestion provided by the test_theme theme.';
-}
-
-/**
- * Returns HTML for a theme function suggestion test.
- *
- * Implements the theme_test_function_suggestions__module_override suggestion.
- */
-function test_theme_theme_test_function_suggestions__module_override($variables) {
-  return 'Theme function overridden based on new theme suggestion provided by a module.';
-}
-
-/**
- * Implements hook_theme_registry_alter().
- */
-function test_theme_theme_registry_alter(&$registry) {
-  $registry['theme_test_template_test']['variables']['additional'] = 'value';
-}