wp-includes/nav-menu-template.php:629Prevents a menu item ID from being used more than once.
$idstring$itemobjectstringfunction _nav_menu_item_id_use_once( $id, $item ) { static $_used_ids = array(); if ( in_array( $item->ID, $_used_ids, true ) ) { return ''; } $_used_ids[] = $item->ID; return $id;}Introduced in 3.0.1. Unchanged from 6.7.7 through 7.1.0.
Signature, return type and hooks compared across 5 parsed releases.
src/wp-includes/nav-menu-template.php, and regenerated for each WordPress release so it tracks the code rather than a snapshot of it.