This bug duplicates your whole inventory, and by the looks of it any item you wish.

It's done by first preparing which item(s) you wish to duplicate, then putting down a chest or barrel. Before putting anything in, you'll need to head to settings and then set the 'simulation distance', 'biome blend' and 'render distance' to max. After that you need to save and quit the game. Load then back into the world, and fill the barrel/chest with the items you wish to duplicate, then while still in the container force the game to quit by closing it using task manager. 

When you reboot the world the items should now both be in your inventory like it was originally, and at the same time in the container.