Index: src/wp-includes/taxonomy.php
===================================================================
--- src/wp-includes/taxonomy.php	(revision 31366)
+++ src/wp-includes/taxonomy.php	(working copy)
@@ -2860,7 +2860,7 @@
 	if ( ! $slug_provided ) {
 		$_name = trim( $name );
 		$existing_term = get_term_by( 'name', $_name, $taxonomy );
-		if ( $existing_term ) {
+		if ( $existing_term && $existing_term->parent == $parent ) {
 			$slug = $existing_term->slug;
 		} else {
 			$slug = sanitize_title( $name );