add_menu_page with different name for first submenu item - http://wordpress.stackexchange.com/questions/66498/add-menu-page-with-different-name-for-first-submenu-item
<?php
add_action('admin_menu','order_admin_menu');
function order_admin_menu() {
add_menu_page('Orders', 'Orders', 'manage_options', 'dp-order/order-list.php', '', plugin_dir_url( __FILE__ ).'icon.png');
add_submenu_page('dp-order/order-list.php', 'Orders', 'All Orders', 'manage_options', "dp-order/order-list.php", "");
add_submenu_page('dp-order/order-list.php', 'Reports', 'Reports', 'manage_options', "dp-order/order-report.php", "");
add_submenu_page('', '', '', 'manage_options', "dp-order/order.php", "");
add_submenu_page('', '', '', 'manage_options', "dp-order/update.php", "");
add_submenu_page('', '', '', 'manage_options', "dp-order/delete.php", "");
}
?>