1) Accept a Bulletin Board request that gives you an item before completing the quest (such as delivery requests).
2) Work upon the request until you receive the item you want to duplicate.
3) Instead of completing the request, go back to the Bulletin Board and cancel the request.
4) The item you were given during the request remains in your inventory.
5) Accept the same request and repeat these steps as many times as you want.
Note: Do not complete the request until you're completely finished duplicating the item.