I found a very simple bug to duplicate slaves. It is not that obvious, so most players probably didn´t encounter it.
1. Move a slave to the 8th slot of an NPC Menu page
2. Open her "Move" Menu
3. Interact with her
4. Close the interaction menu by returning to the npc menu
5. Now the slave in the 7th slot of the menu should be selected
6. Move this slave to another slot
7. Now the moved slave became an exact replica of the first one with all stats, traits and everything. If you modify the original slave (for example changing its traits, willpower or job) the ones of the new slave will change as well.
It should then look like this:
If I explained it well enough, it is really easy to perform. Works in the newest public release version.
If you have any more questions regarding this bug, just ask.