This was tested with two players to determine the root cause, and the answer is the combat logging timer. When a player logs out, their character is forced into the 'sitting' animation. This animation does not allow for two handed items to remain in use. Thus the game forces the character to drop the two handed item in front of them to complete the animation. Thus the 'extra' weapon is deposited on the ground in front of the logging out avatar.
The game -also- adds the extra weapon to the character's inventory to allow them to save that 'extra' weapon through the non-playing time. This results in a duplicate of the weapon that was 'in hands'.
This does not work with protector cases, ammo cans, or medical kits (or any other one handed item that is) because they are allowed to be 'in hands' during the sit down animation and are not forced out of the character's hands during the log off.
All duplicated items were destroyed after completion of this testing by a server restart. However the ability to make these duplications remain.
Also, all duplications maintain all attachments, spray paints, magazines and ammo counts. This means duplication of ammunition and attachments is also possible if you don't want the weapon as a whole.