Had to buy Starcraft and Brood War again because my Blizzard account was compromised.
Bought Mass Effect twice. Once on disc, which I found unplayably buggy, and once through Steam when it was on sale for pennies, in the vain hope that a fully digital, fully updated copy might work. Nope.
Got Doom II and Doom 3 again when I bought the id Super Pack during one of those insane Steam Christmas sales.
Bought Final Fantasy VII again on PSN.
Got several Valve games again because they came packaged with new games: Portal, Team Fortress 2 (back when you had to buy it), Half-Life 2, and probably a couple more. Not sure that counts as they weren't bought so much as free with purchase of other product.
Naturally I have bought games for other people that I also bought for myself. Also don't think that counts.