In Dragon Quest IX for the DS, there is a really simple way to duplicate items.
All you have to do is connect to someone else’s Dragon Quest IX game. Give them the items you want to duplicate, and quit the game without exiting their world. You didn’t save, so when you open the game your items will still be there, and they’ll still have them as well.
Want the item back but have your buddy keep it as well? Have them save their game, then come back and give you the item again the same way you gave it to them. Now you’ll have two, and they’ll have one as well. Keep doing this to duplicate as many as you want.