I've owned 3 different Black Knights, I'm about to buy ST:TNG again, 2 Meteors, and 2 Firepowers. Generally it's because I sell a game when I feel I'm done playing it, then I see a deal on it pop up and I realize I miss it in my collection, so I snap it up again. Usually it's many years apart.
I'm glad to see I'm not the only one that's owned two of the same game at the same time