Index: wp-includes/taxonomy.php
===================================================================
--- wp-includes/taxonomy.php	(revision 14778)
+++ wp-includes/taxonomy.php	(working copy)
@@ -1608,8 +1608,8 @@
 
 	if ( $term_id = is_term($slug) ) {
 		$existing_term = $wpdb->get_row( $wpdb->prepare( "SELECT name FROM $wpdb->terms WHERE term_id = %d", $term_id), ARRAY_A );
-		// We've got an existing term, which matches the name of the new term:
-		if ( is_taxonomy_hierarchical($taxonomy) && $existing_term['name'] == $name ) {
+		// We've got an existing term in the same taxonomy, which matches the name of the new term:
+		if ( is_taxonomy_hierarchical($taxonomy) && $existing_term['name'] == $name && is_term( (int) $term_id, $taxonomy ) ) {
 			// Heirarchical, and it matches an existing term, Do not allow same "name" in the same level.
 			$siblings = get_terms($taxonomy, array('fields' => 'names', 'get' => 'all', 'parent' => (int)$parent) );
 			if ( in_array($name, $siblings) ) {