如何限制在opencart中添加到愿望清单的同一产品?

when user add product in wish list but when user add same product in wish list. At that time i want to give alert message as product already added in wish list.

In which file will change the code.

Please help me.

First you need to check if wishlist is available, then check if product_id is in wishlist array

if (isset($this->session->data['wishlist'])) {
 if (in_array($product_id, $this->session->data['wishlist'])) {
    echo '<script type="text/javascript">alert("Product is already in wishlist!");</script>';
 }
}