Managed To duplicate a Shulker box

So I managed to duplicate a Shulkerbox. Its not a "ghost copy" where the item disappear when you click on it. It can be placed and items can be taken out of it.
I Just broke it with my pic and 2 dropped instead of one
I managed to get the duplicating action on video.
I have not been able to reproduce it since this one time.
I apologize in advance for the bad video but its the best I could do.
Here is the video: https://goo.gl/EfpZFi

Cheers. Marcus