I believe Archie posted this suggestion many moons ago, but it bears repeating on slot card round robins. You HAVE to alphabetize the cards in the package before you begin selecting your replacements!

Nobody can remember the details on 80-100 cards they just looked at, while looking at their own collections to see if they need a card.

Put them in order, and when you put your replacement card in, put it in the proper place in the deck of cards. You'll immediately see if it's a dupe.