diff --git a/core/includes/bootstrap.inc b/core/includes/bootstrap.inc
index 35c9e0c..0c800d0 100644
--- a/core/includes/bootstrap.inc
+++ b/core/includes/bootstrap.inc
@@ -3155,11 +3155,24 @@ function registry_rebuild() {
  * to be called, because it is already known that the list of files in the
  * {system} table matches those in the file system.
  *
+ * @return
+ *   TRUE if the registry was rebuilt, FALSE if another thread was rebuilding
+ *   in parallel and the current thread just waited for completion.
+ *
  * @see registry_rebuild()
  */
 function registry_update() {
+  if (!lock_acquire(__FUNCTION__)) {
+    // Wait for another request that is already doing this work.
+    lock_wait(__FUNCTION__);
+    return FALSE;
+  }
+
   require_once DRUPAL_ROOT . '/core/includes/registry.inc';
   _registry_update();
+
+  lock_release(__FUNCTION__);
+  return TRUE;
 }
 
 /**
diff --git a/core/includes/registry.inc b/core/includes/registry.inc
index 2ddd1f7..9abc3ee 100644
--- a/core/includes/registry.inc
+++ b/core/includes/registry.inc
@@ -156,9 +156,9 @@ function _registry_parse_files($files) {
  * Parse a file and save its function and class listings.
  *
  * @param $filename
- *  Name of the file we are going to parse.
+ *   Name of the file we are going to parse.
  * @param $contents
- *  Contents of the file we are going to parse as a string.
+ *   Contents of the file we are going to parse as a string.
  * @param $module
  *   (optional) Name of the module this file belongs to.
  * @param $weight
@@ -166,17 +166,19 @@ function _registry_parse_files($files) {
  */
 function _registry_parse_file($filename, $contents, $module = '', $weight = 0) {
   if (preg_match_all('/^\s*(?:abstract|final)?\s*(class|interface)\s+([a-zA-Z0-9_]+)/m', $contents, $matches)) {
-    $query = db_insert('registry')->fields(array('name', 'type', 'filename', 'module', 'weight'));
     foreach ($matches[2] as $key => $name) {
-      $query->values(array(
-        'name' => $name,
-        'type' => $matches[1][$key],
-        'filename' => $filename,
-        'module' => $module,
-        'weight' => $weight,
-      ));
+      db_merge('registry')
+        ->key(array(
+          'name' => $name,
+          'type' => $matches[1][$key],
+        ))
+        ->fields(array(
+          'filename' => $filename,
+          'module' => $module,
+          'weight' => $weight,
+        ))
+        ->execute();
     }
-    $query->execute();
   }
 }