Hi,
Yes you can potentially create a little plugin based on the function "onBeforeCartUpdate(&$cartClass,&$cart,$product_id,$quantity,$add,$type,$resetCartWhenUpdate,$force,&$do)".
Do the checks on the product and on the current cart content, and based on these informations set the vairable $do to false to not add the product to the wishlist. You can too use that kind of code to display a message to the customer:
Code:
// The code to check if it is a wishlist
// The code to check if the product is already in the wishlist
// The code to set $do to false if conditions are met
if($do == false){
$app = JFactory::getApplication();
$app->enqueueMessage('Product already in the wishlist !');
}
