Most of them will say something similar to 'How would you like a train set for your birthday?' Answer 'Yes' and you will put in your birthday. Talk to the townspeople until they ask you when your birthday is. When you get to your island, your item will be there again and you will already have it in your pockets. Your island should upload from your Game Boy Advance. When you get back to your town, disconnect Game Boy Advance #2 and connect Game Boy Advance #1 (make sure it is not in sleep mode). This time when Kapp'n asks you to save your island to your Game Boy Advance, answer 'No'. When you get to your island, pick up the item that you left there then leave the island again. After you start leaving you can turn Game Boy Advance #2 off. Talk to Kapp'n to go back to your island. Leave that screen then immediately return to it. When you get back to your town, attach Game Boy Advance #2 and turn it on. After it is done downloading to your Game Boy Advance, you can put it in sleep mode, disconnect it, and set it aside. When Kapp'n asks if you want to save your island to your Game Boy Advance, select 'Yes'. When you finally get to the island, drop off the item or money that you wish to duplicate. Go to your island (located on the dock at the edge of the ocean). Connect Game Boy Advance #1 to the Gamecube using your link cable and turn it on.

This trick requires two Game Boy Advances and a link cable.

Doubutsu no mori translation patch