Machines with export upgrade will duplicate items in the inventory they are supposed to export to for the cost of the processed material.
Miner will do the same for the cost of nothing, or the ore that it mines. I had a miner duplicate cobblestone when it was mining tin and copper ore.

I was playing on my server when I found that.

Mod list:

thanks i will work on that now.

Fixed

It was a dep bug by the way. I check: is the Item a with the Item b equal (and i made it is Item a with Item a Equal)