Issue #2967627 by quietone, yunke: Hook preprocess function always added

(cherry picked from commit 511e9fa1)

core/lib/Drupal/Core/Theme/Registry.php

@@ -754,7 +754,7 @@ protected function postProcessExtension(array &$cache, ActiveTheme $theme) {
     ksort($suggestion_level);
     foreach ($suggestion_level as $level => $item) {
       foreach ($item as $preprocessor => $hook) {
-        if (isset($cache[$hook]['preprocess functions']) && !in_array($hook, $cache[$hook]['preprocess functions'])) {
+        if (isset($cache[$hook]['preprocess functions']) && !in_array($preprocessor, $cache[$hook]['preprocess functions'])) {
           // Add missing preprocessor to existing hook.
           $cache[$hook]['preprocess functions'][] = $preprocessor;
         }