Additional menu item popping in submenus

The third menu item that appears (in fact, the first) is the main menu:

add_action('admin_menu', 'Ad_menu');

function Ad_menu()
{
    //Main menu
    add_menu_page( 
        'Ads page title', 
        'Ads menu title', 
        'manage_options', 
        'ad_menu_slug', 
        function(){ echo '<h1>Main menu</h1>'; }
    );
    //Submenus
    add_submenu_page( 
        'ad_menu_slug', 
        'View page title', 
        'View menu title', 
        'manage_options', 
        'ad_view_slug', // <-- Put main menu slug here
        function(){ echo '<h2>Views submenu</h2>'; }
    );
    add_submenu_page( 
        'ad_menu_slug', 
        'Manage page title', 
        'Manage menu title', 
        'manage_options', 
        'ad_manage_slug', 
        function(){ echo '<h2>Manage submenu</h2>'; }
    );
}

If you change the first submenu slug to ad_menu_slug (the main menu slug), the result will be this: