Destroying a shulker box using a piston or an explosion, or breaking it in Creative mode drops all of the items from the shulker box while not emptying it out, thus duplicating items.

How to reproduce:

Method #1:

1. Place a shulker box and insert any items into it.
2. Place a piston facing the shulker box.
3. Power the piston.

Method #2:

1. Place a shulker box and insert any items into it.
2. Place TNT next to the shulker box.
3. Light the TNT.

Method #3:

1. Place a shulker box and insert any items into it.
2. Switch to Creative mode.
3. Break the shulker box with your hand.