You can currently now duplicate any weapon you have in your hands when having another primary weapon on your back upon logout.

It appears the combat logging timer of 30 seconds is possibly causing this to happen. Whenever your character sits down for 30 seconds prior to logging out, your character will drop the gun he had in his hands on the ground and if you have a friend next to you he can pick it up and it is completely duplicated, ammo, attachments.

If you log back into the same server or another server, you will still have both of your weapons on you as well.