diff --git a/includes/theme.inc b/includes/theme.inc
index cd65d7d..05ea943 100644
--- a/includes/theme.inc
+++ b/includes/theme.inc
@@ -202,7 +202,14 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
   }
 
   // Initialize all themes and engines.
-  foreach (array_merge($base_theme, array($theme)) as $current) {
+  // Make sure we do not load the same theme engine twice.
+  $theme_init = array();
+  foreach (list_themes() as $current) {
+    $key = $current->owner . ' ' . $current->engine;
+    if (isset($theme_init[$key])) {
+      continue;
+    }
+
     if (isset($current->owner)) {
       include_once DRUPAL_ROOT . '/' . $current->owner;
     }
@@ -211,6 +218,7 @@ function _drupal_theme_initialize($theme, $base_theme = array(), $registry_callb
         call_user_func($current->engine . '_init', $current);
       }
     }
+    $theme_init[$key] = $current;
   }
 
   if (isset($registry_callback)) {