You don't have to duplicate the same item. You can have an M4 slung, then take an SKS to hands, and make a copy of the SKS. You do not need x2 of the same item, you only have to make sure the inventory slot it normally would go to is filled already. I.E. Rifle slung slot.
@sheLLHD: I did do a search, and all I got back was people duplicating their own character on high ping servers and other lag related issues. This one is a 100% success rate on any server regardless of ping due to a coding issue introduced in the new patch via the 30 sec log out timer.
The Dev Team rightfully added a code so that weapons carried in hands were saved to a character so that people with two rifles didn't lose them if they logged off. This was never an issue before because when you logged out you simply vanished and the code to add the item to the player's inventory fired.
Now, the code fires to add the item to the player's inventory, and then the 30 sec idle code goes into action. This sits the player down for 30 seconds before vanishing. From there the normal "when playing this animation drop in hands weapon" criteria is met and so a duplicate item is created. It's a small oversight from the addition of the log out delay feature, and one that is quickly and easily fixed by not forcing the idle character to change stance or use any of the emotes.