Index: taxonomy.php
===================================================================
--- taxonomy.php	(revision 17537)
+++ taxonomy.php	(working copy)
@@ -437,7 +437,8 @@
 	if ( ! get_post_type_object($object_type) )
 		return false;
 
-	$wp_taxonomies[$taxonomy]->object_type[] = $object_type;
+	if ( ! in_array( $object_type, $wp_taxonomies[$taxonomy]->object_type ) )
+		$wp_taxonomies[$taxonomy]->object_type[] = $object_type;
 
 	return true;
 }