We're doing something called inverse hypergeometric distribution to check the liklihood of not drawing a single copy of a card given the deck size and number of copies of the card in the deck. When we calculate that probability, we can subtract it from 1 to determine the chance of having drawn more than 0 copies. At least that's how people who know math explained it to me. The whole thing is a lot more fun to say out loud than it is to read about on Wikipedia and you can peep the source code by clicking the fish in the upper right corner.