I hate to admit that I have. I completed the Pokedex in Platinum and I have so much good stuff on that 300 hour save. I really wanted to play through it again, but there's no way I'd want to erase that save, so I bought a new one. For shame, I know.
I bought two Diamonds on accident. First I got just one. I took it home, put it on the table to play it later, then I forgot about it, went back to the store, and bought another one. Felt so stupid when I came home and it was there on the table.
Luckily, I managed to sell one of them for double price to someone. It's still a little embarrassing when I think about it, though.