Now, if the char data is not saved, you can for example give all your items to a friend, or a mule, then make it so that your data is not saved when you log out, then log out, log back in, and voila, items duplication.
2 weeks ago, a player gave us some more info about it it, so we were able to narrow it down a little. After a potential fix (which didn't work), we found the real cause of that bug. It had to do with lagging out while trading. No point to give the exact details, as you can't really understand without knowing how the server code works. But anyway, it was extremely easy to abuse.
Since we didn't know if people knew how to abuse it, we fixed it and didn't tell anyone we fixed it. We just restarted the server and mentioned various other changes, but we did log in to see if anyone was trying to do that.
Fortunately, in 4 days, only 2 players tried to do that, but upon a further inspection (looking at their items and stuff) it seems that it was accidental.
We also did an inventory of the most important items, to see if any player has more than normally possible, and we didn't find any anomaly.