First you must have two controller paks to do this trick and a game note on each.
Start the game normally and go to the Treasure Box menu. Trade to the other pak the item you want to duplicate if it's not there already. Go to the Treasure Exchange option (if not there already). Take the controller pak that has the desired items out of the controller. Continue to trade. If a ''please insert the controller pak back into the controller'' message pops up (either during the trading or after exiting the Treasure Box menu), PRESS B! It will then ask whether you want to use a controller pak or not. say no. Save it just in case by going into the Tournament mode, then cancel out by pressing B. Reset the game. Put the controller pak back in. Voila! You should have a copy of the traded item in both game notes! It's very handy for the legendary items and the Ring of Genie....