Go to a PC, and save. Attach the item you want to duplicate to the Pokemon you want to clone. Deposit it into the PC. Change boxes. When the screen is saying "Saving the game..." Turn off your GB. Turn it back on. You should have the deposited Pokemon in the PC, and a clone of it in your lineup. The item will either be on both monsters, or in your Backpack. Note: while doing this code, DO NOT withdraw any Pokemon. When the GB is turned back on, you'll find that the withdrawn Pokemon has been deleted.

Note I: You can duplicate up to 5 items/Pokemon at a time.
Note II: Use this code at your own risk. It may corrupt your save data if done incorrectly.