To duplicate an item make a pokemon hold the item you want to duplicate. Then with that pokemon diposit it in Bills computer then change box then when its saving and it says SAVING DONT SWICH OF THE POWER tun it off.

To really duplicate item with 100% description here it is. First make sure the pokemon holding the item you want to dupilcate is at the head of your party. Then save the game. log on to the pc and deposit it to an empty box ( for precausionary issues also make sure you are in a empty box before doing any of these steps). When you deposit your pokemon switch to another box. When it says Saving...do not turn off the power, turn it off before that phrase is competed. When you turn it back on you will have a pokemon in your party and the same one in the box with the same item (that is how i duplicated my master balls to catch all dogs and the birds).