Ticket #31270: 31270.patch

src/wp-includes/taxonomy.php

@@ -2860 +2860 @@
         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 );