Duplicating Any Item in Gothic 1

I once saw this in another video (that I can't find again) and somehow believed it to be a trick that only works with Systempack, because the other guy used a quicksave in it and didn't think more about it.
Today I tried to remember it or find the video, but couldn't, so I tried in Gothic 1 with SystemPack 1.8 (standard Steam version) for like 2 hours until I found how to do it (or maybe it is a different method than the one I saw before?). Then I tried it again with my usual Gothic 1 folder and it worked just fine.
I also tested in original Gothic without Systempack or Union.

The method:
Trade the item to be duplicated to a trader
Beat him up
Open his inventory
Close it with B (status screen)
Repeat that a few times
Open your own inventory and close it
Now save the game
Open your inventory and go all the way to the left to move the item over into yours
Close your inventory
Open the trader's inventory again and move the item to your side again