Mainly on items like Triple Triad Cards. I'm not a huge Triad player but do it from time to time. There are times in dungeons like 24-mans where I just can't remember if I have a card so I just roll need only to find out later I have it. If the tooltip showed a message if you already have said card it would me amazing.

The same can be added for Minions, Master gathering/crafting books or anything that's learned only once and can't be learned again.

You already can't roll on mounts you already acquired.