I found a duplication bug. I am playing Enigmatica 2 Expert 1.41, which has DankNull-1.12-1.4.43.jar.

Open an empty dank null's interface, place a block in the first slot, ctrl-click it (to change extraction mode), then still holding your mouse over it, press your drop key. The block will be dropped out of the dev null. Then hit escape; the block will reappear in the dev null (if you reopen the interface you can take it), but also the dropped one will be on the ground. So now you have two of the block. They are both real items.

As pointed out in #143 you can also just pull the items out, don't need to specifically hit the drop key.