Index: t3lib/cache/backend/class.t3lib_cache_backend_memcachedbackend.php
===================================================================
--- t3lib/cache/backend/class.t3lib_cache_backend_memcachedbackend.php	(revision 7645)
+++ t3lib/cache/backend/class.t3lib_cache_backend_memcachedbackend.php	(working copy)
@@ -484,7 +484,7 @@
 
 					// Update identifier-to-tag index
 				$existingTags = $this->findTagsByIdentifier($entryIdentifier);
-				if (array_search($entryIdentifier, $existingTags) === false) {
+				if (array_search($tag, $existingTags) === false) {
 					$this->memcache->set($this->identifierPrefix . 'ident_' . $entryIdentifier,
 						array_merge($existingTags, $tags));
 				}