You and me both.
The majority of my dupes are Resident Evil games:
I got the original Resident Evil on Playstation, then Directors Cut, and of course, RE: Deadly Silence is essentially the same game but for DS (although with a few extras). I got REmake when it first came out on Gamecube and picked up RE Archives just this week. Cheapest port ever, because there's no additions... hell, it doesn't even have widescreen support!
RE2 I first played on Gamecube, but was intruiged to see what the original version was like so I also got it for Playstation. Then I wanted to see what the N64 version was like, as it's held up as one of the best technical achievements on the N64. It's obvious they put a lot of effort into the port but those cut scenes are heavily compressed and just don't look very good.
Code Veronica I had only played on Dreamcast so I wanted to see what Code Veronica X was like. RE4 I owned on Gamecube first and was tempted by the extra content of the Ps2 version. However, since the Wii version has all of that, the PS2 version doesn't seem so tempting now.
I really enjoyed Shadow Man when I first played it on the N64, so I eventually ended up getting the, visually much sharper, Dreamcast version. I seen the Playstation version in really good conditon so I just picked that up as well.
GTA III - I never played this on the PS2 when it first came out, but I was blown away when I played it on Xbox. The PS2 version is something I seen just recently in excellent condition and very cheap, so I decided to get it.
NFS Hot Pursuit I first played on Gamecube... I enjoyed it despite its obvious faults (and because at that time the Gamecube was my only console and there's weren't many motor racing games available for it). When I got a PS2 I was curious to see what the superior version of the game was like.
Dead Rising... I loved the 360 game. It was really THE game that made having a "next gen" console worthwhile at the time. So, I was curious about the Wii version, despite the bad rep it was getting. It's true there's less zombies on screen but there are other changes that make this less obvious (open areas are reduced in size) and there are new enemies and a much better targeting system. I enjoyed playing through both versions of the game.
I've never gotten around to selling the standard version of Morrowind, but I might just keep it since the artwork is different. That sounds silly, I guess. I've owned the original Ninja Gaiden (on Xbox) then traded it in for Ninja Gaiden Black. Now that I've got Sigma I'll probably sell Ninja Gaiden Black (stupid Classics/Greatest Hits cover means I don't have much love for that one).