I am having some... interesting experiences with this.

I use an Ender Chest as a backpack that players should not be able to move in their inventory. It works fine in survival and adventure modes. However, if in Creative Mode, this becomes really glitchy. If I click on the ENDER_CHEST (which should not move from my inventory), the item is suddenly duped. The duped materials can only be dropped, but never placed back in the inventory.

This isn't all. If you take other items, let's say 32 diamond blocks, and click on this item, then they suddenly are destroyed by the game, and the duplication of the ender chest happens again.

I included below the plugin in which causes this, as well as the code that I am using.

I want to note that even if all I have for the event in event.setCancelled(true) and nothing else in the method, these weird glitches still occur.