I already posted this thread on Giantbomb but I'm interested in seeing your answers!
Ever since I got my 360 over a year ago I've been periodically re-buying games I already owned on Steam and PS3 with no intent of trading the old copies in. It started off with multiplayer games which are easily justifiable since I could then play with my friends who didn't own the game elsewhere. But then I started buying a few single player games, mainly due to sales on XBLA or Steam. So far my list consists of Super Meat Boy, Bastion, Mirrors Edge, Modern Warfare 2, Bioshock 2, Street Fighter IV along with a few others. Maybe I've been bamboozled by low low prices into buying something twice, but I don't feel too guilty about it.

So my question to you all is; what games have you bought more than once and why?

I wouldn't re-buy a game that is, for all intensive purposes, exactly the same. BUT, I do own multiple copies of various games. Usually, it's because one is mobile or in a more convenient format. But, sometimes, they're just better/different on different systems.

I've also re-bought some games purely because the version I had wasn't the best experience of it. But, it's mostly older games ... and this generation is sort of the first generation where almost regardless of where you get a game it'll pretty much be the same experience ... that wasn't the case before now.

Vanquish on PS3 is the same as on 360. But, Mortal Kombat 1 is not the same on Genesis and SNES. Or Resident Evil on PS1 and Saturn. etc.

I understand in general ... but I don't understand unless there's a reason. Them being cheap ... isn't a reason

Weirdo!

I bought Fallout 3 for the PC. After all the DLC was released, I opted to buy the Game of the Year version for PS3. Not sure what compelled me - but at the time I think it was only a few dollars more for the bundle versus buying all the DLC individually.

I've also been known to buy old PS1 games for PSN when I already have the discs.

I'm worse about buying several versions of the same movie. For instance, I have Dune on VHS, DVD (which was my second ever DVD purchase), the special edition in the metal case, and on bluray.

Once I realized I spent money on 4 versions of Dune, I've since been a little more cautious about my entertainment expenditures.

If you're buying multiple versions for convenience sake, like mobile platform for your commute and console for relaxation, within reason, then you're fine. Buying it for the hell of it is.....well....I own 4 versions of the same movie so I can't judge.

Usually as replacments for broken or lost copies.

Oh ... I have it on Sega CD too and I intend on buying the Arcade version for PS3 (ARCADE PERFECT! Finally. Jerks.)

The only thing I could think of is maybe I'll re-buy a game on PC if I bought it on PS3 or something. But this is usually because the experience is significantly different ... or cheaper than buying DLC

For example: Playing Street Fighter IV Arcade at Ultra HD settings with full AA on a massive LFD monitor would be infinitely more pleasant than playing it with a controller on the couch. <~ just an example!

I remember an especially nerdy moment when I bought the Gold Versions of the original 'Legend of Zelda' and 'Ocarina of Time' when I already had the regular gray versions. I kept the gray versions for a while until I realized they were screwing up my game count, so I traded those in.

I wouldn't mind buying two different versions if they were different games. For example, I heard 'Aladdin' on the SNES and Genesis are two different games, so I might buy both copies.

Granted, with my obsession of Final Fantasy, I own Final Fantasy Origins, Chronicles and Anthologies for the PS1 when I already own the original carts. But, I consider those remakes instead of the same game. For example, Final Fantasy II (NA) is completely different from Final Fantasy IV (Japan).

I don't download games, but I kind of don't see the point of doing it when you own the exact same game. That seems like a little waste of money.

Different Game, Same Name - see the Aladdin example, this happened a lot in the SNES/Genesis era.

Port - Sometimes it is better to own it on a newer console or handheld rather than hooking up an old system or a certain system has some exclusive content (like the extra levels and character in the Saturn version of SotN)

Remake - I think I have picked up FFIV like 4 or 5 different times but each release is different enough upgrade

Multiplayer - I already own Guardian Heroes but someone on my friend list wanted to play co-op so I bought it again

But I would find it hard to justify buying the same game that I already have on 360/PS3/PC on the others. There are too many games I don't already own, that I would rather buy and the differences between the three are waffer thin.

Well I have two copies of MW3 so I can play with mt 360 and PS3 buddies... although I barely play it on PS3.

I have been known to get a game on 360, trade it in to get something new and when the price drops to mega cheap I get it on PS3. Done this with Batman Arkham Asylum, Assassin's Creed 2, Bioshock, Dead Space and GTAIV (got the complete edition so I can play the main story as well as the DLC which I haven't played through entirely on 360).

I only buy one copy of the games, unless the remake or port is a different experience. I have Super Street Fighter II on the SNES, but I wouldn't feel bad buying Turbo HD Remix, because it has an online component, is completely redone graphically, and plays like the arcade version (plus I could use my stick instead of an annoying d pad). Other than that, things like achievements or simply the convenience of having it downloaded on my PS3 or Wii won't serve as the basis of rebuying the same game.

When it comes to games with online multiplayer, I always end up double dipping. There's always at least one friend I'd be willing to go to the other console for, even when all the rest are on another. Aside from that, it's rather rare for me to buy something on multiple consoles unless I really enjoyed it.

Counter Strike.

Fucking CD-keys

Counter-Strike was free even after retail release

Lolnope. Counter Strike came with Half-Life Generations pack, which needed a CD-key to be played. Lose the CD-key, need to rebuy Generations.

No, you just needed a Half-Life 1 key. It didn't need to be specific to generations, collection, or anthology ... or the retail copy of Counter-Strike.

I knew a few people who couldn't hang onto HL1 keys for whatever reason, I never really understood why it was so difficult to not throw away the jewel case or the slips ... or even the entire box.

Oh yeah fu(< CD Keys

Yeah that stuff is easy to lose. You take your stuff over to someone's house and they think it is theirs or you move a couple times and that kind of stuff always seems to slip through the cracks.

I actually found my old Diablo II disks in storage but I was missing the expansion disk so I ended up rebuying the entire battlechest because it was cheaper than just rebuying the expansion disc alone. One of the times I know I wholesale bought a game again.

I tend to repurchase games for iOS and PC. 

For iOS its usually that I really like a game, but I find that the play style is better suited for several short rather than few lengthy game sessions.

Plants vs Zombies, Super Crate Box, Greed Corp, Puzzle Agent are only to name a few iOS games I've bought for this reason.

As for PC games, the usual culprit is Humble Indie Bundle. I've rarely bought a bundle where I didn't already own one or two titles. In a way I guess it doesn't count since I'm buying the bundle for the games I don't have. But in a couple of cases I have bought it in order to get the Mac version of a bundled title that I already own on for Windows.