Item duplication. And items crafted with duplicated items (door, sign, e.g.) when placed disappears and when you look on your inventory you will see the normal number of the items.

I joined an external server. A player in the game thought me how to do it.
Steps to Reproduce
1.) Click your chest
2.) Put the item to be duplicated in the chest
3.) Put it back again in your inventory
4.) Do it repeatedly
He said it works with 2 or more players. It works on external. Not mentioning Realms and Local Multiplayer because I haven't tried them. Please fix this bug to make this game fair.