I have 3 copies of Ocarina of time, but 2 of those are special edition gamecube bundles with other games and one is the original N64 cartridge.
For my birthday my dad and my then-boyfriend both bought me Bioshock 2. :D
And I've had to buy a replacement cos I've lost one, then found the original so ended up with two..
But I don't think I've accidentally bought replicates. I usually recognise whether I own the game or not.