Instructions
Capture any Pokemon in "Sapphire." If you wish to duplicate items multiple times, capture multiple Pokemon and store them in the Pokemon Center PC for later use. Keep at least one of these Pokemon in your active party. Low-level Pokemon are ideal, since they take little effort to capture.
Enlist your friend's assistance. He can turn on his Pokemon game and attach the item you want to duplicate to one of the Pokemon in his party. Connect the systems together using Game Boy Advance link cables. Both players should speak to the Cable Club representative in a Pokemon Center to initiate a trade.
Select the Pokemon you captured in Step 1 to trade. Your friend will choose the Pokemon with the item attached to trade. Accept the trade. Once you receive the Pokemon holding an item, save your game. Once your friend receives the Pokemon you traded, she should immediately turn off her system without saving.
Turn on your friend's system again. You will both possess the Pokemon holding the item, and the Pokemon that you originally captured will not appear in either game. Repeat this process multiple times to receive infinite numbers of items.