Admin adding submenu that has the same destination as the parent menu

the plugin Site Kit by Google plays with the capabilities to do that. you can see that in the source code here.
this works actually with WordPress 6.8.1 but it’s not sure this trick will work with next versions.

to use the same trick in your code, you have to change the 2 parameters with comment in this code :

add_action('admin_menu', 'top_level_menu', 50);

function top_level_menu()
{
    
    $menu_slug = 'wpdocs-orders-slug';
    
    add_menu_page(
          'WP Docs Orders'
        , 'WP Docs Orders'
        , 'do_not_allow' // this hides the first sub menu
        , $menu_slug
        , false
    );
    
    add_submenu_page(
          $menu_slug
        , 'Existing WP Docs Orders'
        , 'Existing WP Docs Orders'
        , 'read'
        , "wpdocs_orders_function" // use a different slug than the parent
        , 'wpdocs_orders_function'
    );
    
}