I was able to accidentally dupe an MP5 mag. This bug does apply to other weapons as well though. It seems like when the server is lagging the mag is not removed from the weapon when removed from the weapon. I had an instance where I did that and suddenly had a mag in the weapon and the same mag in my backpack. I was able to reload the mag while it was in my backpack without having to shove it back into the weapon - while it was still in the weapon at the same time! the mag was like mirrored. At some point when trying to detach it from the weapon I ended up with other instances of the mag and some of them would remain (duped).