Item duplication glitch with shulker box

So me and my friend were messing around in the Vanilla 1.12 server that we play on. (both of our clients are untouched 1.12 too). And we found out that if one of us have the shulker box opened and the other one breaks it (the person who has it open have to take the items out while server is lagging) items duplicate. We tried it multiple times and it worked. I am gonna include YouTube video because it too big to upload it here.

https://www.youtube.com/watch?v=EVL0oySDGhc