There are two ways to dupe items working right now that I know of, one of which allows for duping a main weapon and the second one - your whole equipment.

The first one is basically done like that - you take one weapon on your back and the second one in your hands, then you log out. A copy of the weapon you hold in your hands is dropped on the ground, however it also stays in your inventory when you log into the game again.

The second one is a bit more complicated. When a person logs out, it takes some time before his/her character actually disappears. During this time you can handcuff them, allowing for his/her inventory to be accessed by other players, who can then take whatever from their inventory they want. If the player is unhandcuffed before his/her character disappears (30 seconds since disconnecting), when they log back in all their items are still in their inventory. This allows for duping of players' entire inventory.

I think that both of these bugs should be fixed ASAP, as they affect the gameplay a lot.