You can duplicate items if you have a chest and you are playing with one other person.
Place everything you want or need duplicated into the chest.
Person who owns the world save!
Friend remove everything from the chest to be duplicated.
Friend save your player, Owner do not save yours!
Exit to title screen then restart your world.
Check out your chest if done right then everything will be in there and in your friends inventory.
WORD OF WARNING!
IF you do not follow this exact order of saving per person, you could lose everything you wanted to duplicate.