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There's another method that works for me.. All I do is put the item I want to duplicate into a chest then ask a friend/whoever to go into the chest as well. Once you've put it in the chest what you want to do is hold on to the item so it goes in to your inventory. When the bar above the item is half way full with green ask your friend or whoever you're playing with to also hold the item, you have to hold onto it until it goes into their inventory, make sure your friend/whoever does the same. Then you put the item back into the chest and tell you're friend to do the same and once both of you have put it in, the amount will have increased. It may sound confusing but it's really easy. *Disclaimer: May not work with all devices.*