Instructions
Visit Battle Frontier and go to the Battle Tower after you have acquired a Master Ball. Choose a pokemon to clone and attach the Master Ball to it. Go to the computer and log on to "Lanette's PC." Select the pokemon you chose earlier and deposit it into an empty box.
Log off of the PC and save your game. When the save process completes, log back on to Lanette's PC. Withdraw the pokemon you just deposited and log off of the computer again.
Speak with the blue-haired woman at the desk to the far right. Choose any two pokemon in your list for a fight. The game will freeze briefly and you will be prompted to save before entering the battle room. Do not save your game.
Turn the GBA off and back on. When you go to your game save, the pokemon you withdrew will be in your party. Log back on to Lanette's PC and select the deposit box you used before. The same pokemon will still be in the box and both will have Master Balls.