The curious thing about this is that the weapons share ammo (i.e. 5 shots from either gun will empty the other), attachments and other things. So if you remove the scope from one, it removes from both. Unload one, they both unload, etc.
The item will work but if the dupe'd one (i.e. the person who was knocked out) drops the weapon it does indeed disappear.
Original bug was found while in town of Balota within arms reach of the water pump. Was harder to recreate outside of that small radius, less frequent but did still occur outside of there. Dunno if it has to do with the water pump but it was easier to recreate at the water pump