It is not clear from product details page whatever product you are about to add to wishlist is already added or not, unless you do it and popup will tell you. How to make a condition
responsive / templates / addons / wishlist / hooks / products / buy_now.pre.tpl
{if !$hide_wishlist_button}
{include file="addons/wishlist/views/wishlist/components/add_to_wishlist.tpl" but_id="button_wishlist_`$obj_prefix``$product.product_id`" but_name="dispatch[wishlist.add..`$product.product_id`]" but_role="text"}
{/if}
if this button was named watch, then after clicking watch it would become unwatch ?