Instructions
Prepare the Pokemon you wish to duplicate by capturing it, training it to the level you desire and teaching it whatever moves you want to be copied. Let it hold any item you also wish to copy.
Go to the Battle Frontier and enter the Battle Tower. Deposit the prepared Pokemon into the PC. Exit the PC and save your game.
Withdraw the Pokemon into your party. Make no other changes to your party or your stored Pokemon.
Talk to the attendant on the far right of the Battle Tower (closest to the PC) and choose any two eligible Pokemon for the battle. Say "yes" when she tells you that your progress must be saved before beginning the battle; a long pause will occur as the game saves your party information, but it hasn't yet updated the PC save information.
Wait until the attendant asks if you'd like to save. Turn off or reset the game without answering.
Turn the game back on, load your saved game and verify that you still have the prepared Pokemon in your party. Go to the PC and the Pokemon should also be in the same box and position that you put it earlier, giving you an exact copy of it.