Wordpress add submenu

add_menu_page('My Custom Page', 'My Custom Page', 'manage_options', 'my-top-level-slug');
add_submenu_page( 'my-top-level-slug', 'My Custom Page', 'My Custom Page',
    'manage_options', 'my-top-level-slug');
add_submenu_page( 'my-top-level-slug', 'My Custom Submenu Page', 'My Custom Submenu Page',
    'manage_options', 'my-secondary-slug');

Source: developer.wordpress.org

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