Index: wp-includes/taxonomy.php
IDEA additional info:
Subsystem: com.intellij.openapi.diff.impl.patch.CharsetEP
<+>UTF-8
===================================================================
--- wp-includes/taxonomy.php	(revision a814ef7bcc08b73956da698ebf6555f7d9daea28)
+++ wp-includes/taxonomy.php	(revision )
@@ -3352,15 +3352,22 @@
 	 */
 	$parent = apply_filters( 'wp_update_term_parent', $args['parent'], $term_id, $taxonomy, $parsed_args, $args );
 
-	// Check for duplicate slug
-	$id = $wpdb->get_var( $wpdb->prepare( "SELECT term_id FROM $wpdb->terms WHERE slug = %s", $slug ) );
-	if ( $id && ($id != $term_id) ) {
-		// If an empty slug was passed or the parent changed, reset the slug to something unique.
-		// Otherwise, bail.
-		if ( $empty_slug || ( $parent != $term['parent']) )
-			$slug = wp_unique_term_slug($slug, (object) $args);
-		else
-			return new WP_Error('duplicate_term_slug', sprintf(__('The slug &#8220;%s&#8221; is already in use by another term'), $slug));
+	// Terms with duplicate names are not allowed at the same level of a taxonomy hierarchy.
+	if ( $exists = term_exists( $slug, $taxonomy ) ) {
+		$existing_term = get_term( $exists['term_id'], $taxonomy );
+
+		if ( $name === $existing_term->name ) {
+
+			if ( is_taxonomy_hierarchical( $taxonomy ) ) {
+				$siblings = get_terms( $taxonomy, array( 'fields' => 'names', 'get' => 'all', 'parent' => $parent ) );
+				if ( in_array( $name, $siblings ) ) {
+					return new WP_Error( 'term_exists', __( 'A term with the name and slug already exists with this parent.' ), $exists['term_id'] );
+				}
+
+			} else {
+				return new WP_Error( 'term_exists', __( 'A term with the name and slug already exists in this taxonomy.' ), $exists['term_id'] );
+			}
+		}
 	}
 
 	$tt_id = $wpdb->get_var( $wpdb->prepare( "SELECT tt.term_taxonomy_id FROM $wpdb->term_taxonomy AS tt INNER JOIN $wpdb->terms AS t ON tt.term_id = t.term_id WHERE tt.taxonomy = %s AND t.term_id = %d", $taxonomy, $term_id) );