$factory->register( 'skin1', 'Skin1', $callback );
$factory->register( 'skin2', 'Skin2', $callback );
$names = $factory->getSkinNames();
$factory->register( 'skin1', 'Skin1', $callback );
$factory->register( 'skin2', 'Skin2', $callback );
$names = $factory->getSkinNames();