I frequently forget whether I already have four Tundras or whatever by the time I'm actively trying to duplicate cards, and wasting the choice on a fifth one I can't (or won't, if I have a Tome of Enlightenment) put in my deck is frustrating. It should show the number already owned, either in the upper right of the card as in the deckbuilder, when it's confirming whether to take this card after you click on one, or both.