First you need more than 9 of an item. (99 is the most effective) Start a new game plus in "Inferno" mode and carry over all your items from the lower difficulty. "Inferno mode only allows you to hold 9 max items, however the items that you had over 9 of will still be over 9 when you start the game. Now open a chest that contains the item you want to duplicate.(let's say Str Elixer) You'll get a message "The chest contains 1x Str Elixer but you already have -90". Open the chest again and you'll see "The chest contains 91x Str Elixers but you already have -90". If you keep opening the chest it will keep adding the excess item you have over 9 to the chest. Afterwards, you can just open the chest and refill your item to 9 as long as there are items left in the chest. This works great with Stat boosters. Be carful not to fill the chest past 999 or it will fix your inventory to 9 of that item and the chest will revert to only giving you 1 item again.