Instructions
Go to a Pokecenter with a Pokemon storage PC and save your game.
Give the item that you want to duplicate to a Pokemon in your party.
Put that Pokemon into an empty storage box on the PC.
Switch to a different box on the PC and the game will save.
Turn off the game when the message ̶0;Saving the game̷0;̶1; appears on the screen.
Turn the game back on. You should have a copy of the Pokemon and the item it was carrying in your party, and one still in the PC storage.