How do you clone on Diamond?

Go to the GTS building. There deposit the Pokemon that you want to clone. Make an offe that no one would trade for. When the clock in the bottom-right corner does 8 1/4 to 8 1/2 turns, turn off and on the game. You will get the "corrupt save" message. this is good. Withdraw the Pokemon, and you then have two of it.

This works since the Pokemon is saved on the Nintendo server, and because you turned off and on, the missingness is not saved on your game.