New instance: in the northern section of level 2 where the grey goblins attack. I had recently loaded my game. I fought the goblins and picked up the gold they dropped and put that pile into one of my containers. Then I went to one of the alcoves that had one of the other goblins, picked up the red gem there, and put it into a separate container. However many coins I picked up, the gems would become that amount.
I was able to do something similar with objects from my own inventory. I dropped a pile of coins, then dropped a gem. I picked up the coins and put them into a container in my inventory, then did the same with the gems. The gems were duplicated.
So far it only seems to duplicate light items like keys, gems, potions, leeches, etc.