diff --git a/wp-includes/taxonomy.php b/wp-includes/taxonomy.php
index 3ed69bfdbb..1d368e3359 100644
--- a/wp-includes/taxonomy.php
+++ b/wp-includes/taxonomy.php
@@ -354,7 +354,7 @@ function register_taxonomy( $taxonomy, $object_type, $args = array() ) {
 	unset( $args['capabilities'] );
 
 	$args['name'] = $taxonomy;
-	$args['object_type'] = (array) $object_type;
+	$args['object_type'] =  array_unique( (array)$object_type );
 
 	$args['labels'] = get_taxonomy_labels( (object) $args );
 	$args['label'] = $args['labels']->name;
@@ -437,7 +437,8 @@ function register_taxonomy_for_object_type( $taxonomy, $object_type) {
 	if ( ! get_post_type_object($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;
 }