Oh man, with retro games vs collections it's a big thing. For example, I bought the Mega Man and Mega Man X collections even though I own all the games on their original systems.
I bought three copies of Darksiders for whatever reason (Xbox, Onlive (ugh), PC).
Rebought a ton of games after getting a PC, most notably Skyrim, Devil May Cry 4, etc.
I bought around five copies of Nier because I wanted the company to have money and they went under anyway. I later gifted the extras to friends (It's an amazing game, yo).
P3FES and P3P.
I've bought Resident Evil 4 on every system it's come out on, whatever that says about me.
Switching to PC exclusivity was the smartest thing I did, though, because it cuts back on dupes and the games tend to be a lot cheaper. Replacing the games from other systems is also less (since most games drop fast on Steam sales).
Oh, I've bought Phoenix Wright like three times because I KEEP LOSING IT. Ugh.