Instructions
Fly to the Battle Frontier or sail there using the S.S. Tidal via Slateport City or Lilycove City. Enter the Battle Tower.
Equip any Pokemon with the item you want to clone (the Pokemon must be in your party). It's ideal to choose a Pokemon you want to clone as well because the trick duplicates both the Pokemon and the item it's holding.
Go to the PC in the Battle Tower, access Lanette's PC and make sure no Pokemon are in Box 9. If any are in there, move them to another box.
Deposit the Pokemon with the item into Box 9.
Press the "Back" button to exit the PC. Then press "Start" and save the game.
Open Lanette's PC again and withdraw the Pokemon from the box, adding it to your current party.
Talk to the lady in charge of the Link Multi-Battle Room (she's found to your right near the PC). Select "Challenge" and choose "Open-Level."
Select two Pokemon other than the one with the item you want to duplicate.
Confirm that you want to save and turn off the game during the pause before the game is actually saved. Be warned: Doing this could corrupt your game data.
Restart your game and choose to continue. The cloned Pokemon will be in your party with the item, and another will be in Box 9, identical to the one in your party.