Chest duplication glitch

If you place down a chest, save and quit, log back in and then quickly put the items you want to duplication in mine 6 blocks of dirt in next to it for some reason? Then you quit and restart Minecraft then join back into the world items are duplicated.

https://www.youtube.com/watch?v=TzNsIwAYQ2U (not my video time stamp 4:21)