This is pretty simple to explain and reproduce: Dollies stack, and if you pick up a chest (or anything with an invetory that the dolly can pick up) with a stack of dollies, you can split them into individual dollies, each with their own copy of the chest, complete with inventory.

I guess a simple fix is to make dollies unstackable. A workaround for now is to disable the dolly in the config, which is what we did on our server after some crafty players found out about this one.