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Depends on the game. On red, blue, yellow, crystal, ruby, diamond, and pearl you can't. On Gold and Silver you go to a pokemon centerand save before you duplicate, place said pokemon in an EMPTY BOX, go to change box and select any, while it saves, turn off game. On Emerald you have to be able to reach the Battle frontier adn you go to the do the same except you place pokemon in computer in the battle tower then go to the lady behind the counter closest to you and you go with what ever she says and when it says the game is saving, turn it off.

Well you basically can't duplicate in diamond or pearl but... If you have 2 Ds you can like get the three starter pokemon easily.

i don't how to, but here a few that i'm pretty sure do 1www.gamespot.com, 2www.cheatcodes.com,3www.cheatcodecentral.com. try 1 first.