Have you ever bought the same game on multiple systems?

Need For Speed: Most Wanted and Burnout Revenge. I had the PS2 versions of the games years ago, and I just bought them for 360 almost three weeks ago.

Yep, did it with Max Payne 2 (PS2 version, then later PC), and The Orange Box (PC versione, and then later I bought the X360 version).

I had the 360 versions of Eternal Sonata and Dynasty Warriors 6.  After I got rid of my 360, I bought the PS3 versions of both.

I have never done that. 

I have Lost Planet, Devil May Cry 4 and Gears of War on console and PC.

Yup. I own Mass Effect on both the PC and 360. I bought the CE of Bioshock for the 360 and I bought it for PC as well. Orange Box and Grand Theft Auto 4, same thing.
 
I also own a boxed copy of Crysis, and a Steam version of Crysis.

There's a lot of games I end up rebuying on steam when they become cheap. 

THPS2 on Playstation and on GBA. Not sure if that counts though. lol

I purchased MW 2 for my PS3 when I thought my Xbox was going out....it didn't.

I have Crash Bandicoot: The Wrath of Cortex on the PS2 and GameCube. I like to think it was an accident. 

I have, but it's usually 360 games that I also bought for the PC. These include Bioshock (can't beat $2.99 sale on Steam), Fallout 3, Oblivion, Mass Effect 2, and Resident Evil 5.

BioShock - PC, 360 
Jade Empire - PC, XBOX
Mass Effect - PC, 360 
Mass Effect 2 - PC, 360 
The Orange Box - PC, 360

Orange Box and Oblivion on both Xbox and PC. Eventually sold both the xbox versions though.

I have Marvel vs. Capcom 2 for the original Xbox, as well as XBLA. 
 
Other games I can think of (Most attributed to a steam sale or something): Mass Effect, Fallout 3, Braid, Morrowind, Oblivion.. 
 
Better yet are games I have bought more than one of for a single system. Some are disk issues, but some others were because they were cheap and I want them for system linking. 
 
Those are:  
Unreal Championship (Xbox) I had 5 copies that I paid $5 each for, now I have two. I gave the others away at a LAN party. 16 player Unreal for $20 was nice. 
DOA4 (X360) I had one disk kind of die on me, works about 1/3 of the time. 
Splinter Cell Double Agent: A couple multiplayer maps on one of my disks don't work. 
Saints Row 2: I love this game, and have personally purchased 4 copies of the game. Two of which I sold to friends, and two I have sitting here. This way I can play Co-op with my other box over system link. My girlfriend likes the game quite a bit. 

I have the orange box on 360 and I own the games in the orange box on pc (separately).

Only XBLA & WiiWare virtual console stuff.
I have at least a dozen games on those that I already have sitting in my other room for their original systems.

Orange Box.  That was such a great deal I couldn't have passed it up.

Burnout 3 (xbox, Ps2, xbox 360 download)
Burnout Revenge (xbox, 360)
Burnout Paradise (360, PS3)
Puzzle Quest (PSP, XBLA)
Call of Duty 2 (PC, 360)
Call of Duty 4 (PC, 360)
Doom 3 (Xbox, PC)
Doom (SNES, PC)
Peggle (PC, XBLA)
Orange Box (PC, 360)
Counter Strike (Xbox, PC)
Far Cry Instincts (Xbox, 360)
Battlefield 2 Modern Combat (Xbox, 360)
Left 4 Dead (PC, 360)

Bought XIII on PS2, PC and Xbox
GTA: San Andreas on PS2, PC, and Xbox twice
The Orange Box on 360 and PC

all the time. Street Fighter IV, Modern Warfare 2, GTA San Andreas, GTA Vice City, GTA III, Freedom Fighters  
 
well, the list goes on 

zombie uprising 
 
and yes

I bought the original Mass Effect a few times. First on the 360, then twice on the PC (EA store sucked). When Mass Effect 2 rolled around and I still hadn't played the original to completion and I couldn't find my 360 version I preordered a new 360 ME together with ME2 so I could make a savegame and carry it over. That's two systems and two copies/system so I believe thats the most times I've bought the same game. 
 
A few times come close though but I just can't think of them right now. I bought Ninja Gaiden for the original Xbox which I repurchased when the black edition came out and then, after loosing that, repurchased another black edition to play on my 360 when I had a NG itch to scratch.

Fallout 3 for PC, for steam, and then for the 360....Stupid stupid move..lol

SFIV was the only game that comes to mind right now. Have it for 360 and PC.

Left 4 Dead on PC and 360. I have the Orange Box for 360, but I also have TF2 and HL2 on my PC. 
 
I also have Splinter Cell: Chaos Theory on the PC now, which I used to own for the Xbox.

Madden 07 on 360 and Wii, and RE4 on GC and Wii.

Yeah Morrowind...got it for PC for free, bought it for Xbox then bought it for PC again.
And Oblivion, got it as a gift on 360 then bought it for PC.