add multipe sub menus using add_submenu_page

a loop is almost always a good solution to avoid to copy-paste code.

you just need to use a array of datas like that :

$submenus = [
    [
        "page_title" => "page1",
        "menu_title" => "title1",
        "slug" => "wp-admin.php",
    ],
    [
        "page_title" => "page2",
        "menu_title" => "title2",
        "slug" => "wp-login.php",
    ],
];


foreach ($submenus as $value) {

    add_submenu_page(
        '/pluginname/includes/admin-menu.php',
        $value["page_title"],
        $value["menu_title"],
        'manage_options',
        $value["slug"],
        '',
        plugins_url( 'pluginname/images/icon.png' )
    );


}