Cheat

Easier Item Duplication
Get in a fight and have the order go: Rudy, Jack, Cecelia Have Rudy use a heal berry on anyone Have Jack use a heal berry on anyone Have Cecelia switch the heal berry with what you want to duplicate Fight as usual (If it doesn't work repeat the prosses and switch the heal berry with the space where the item should be)

Item Duplication
1. Have characters in battle. Have it go, Rudy, Jack, Cecilia
2. Have Rudy use heal berry on anybody.
3. Have Jack switch heal berry with item you want to duplicate (Make sure you have only 1 of that item). Defend
4. Have Cecilia defend.
5. Have Rudy use heal berry again.
6. have jack do the same thing as last time, even though there is an empty space where your item was.
7. have cecilia defend.
8. Voila! You now have 255 of that item!

(Note: Make sure you don't kill your enemy with counterattacks, or you'll lose that item for good.)