I have a few duplicates. Obviously, remakes don't really count. I don't think compilations do, either. Final Fantasy IV is a part of both categories. The SNES, DS, and PSP versions are all different enough that owning copies of each isn't a big deal.
Then there's "Game of the Year" versions, or whatever, where oftentimes getting those copies is the same value or better than getting all the DLC. So for games like Borderlands and L.A. Noire I have two copies just because I already had the base games when they came out, but never got the DLC. The GOTY editions were waay cheaper later on than getting the DLC individually.
I also have two copies of Baldur's Gate: Dark Alliance, one on PS2 and one on Gamecube. This is because my PS2 would no longer read the disc. I didn't just get rid of the game though because it's the PS2, not the disc. So I just snagged a Gamecube copy at a yard sale for probably about $1.
Otherwise I don't bother with duplicates. I don't care about keeping a copy sealed and using another to play, or anything like that.