Have a rifle on your back & the rifle you wish to dupe in your hands.
When you go to log off you character places the 2nd rifle (one in hand) on the ground at its feet as it sits for the 10-15 second log off timer.

Another player can pick up this gun thus Duping it.

When you log back into another server you still have the gun & so does the other player.
We have tested this a few times Fri/Sat this week. Mosin, SKS & M4.
Gun, Attachments, Ammo & Clips all dupeable.

A few odd occurrences that could be possibly combating this duping.

After several hours of using a dupe gun.
We noticed that after about an hour of running around with the guns.. One of us dropped the gun & it never spawned.. Waited for it for several minutes. Dropped the other gun and it spawned instantly. So perhaps once the Duped gun is dropped again after some time it is finally deleted?

We tested this again later on and had the same thing happen again. After a good hour or 2 of using a dupe gun.. IT was dropped on the ground and it disappeared.