I am pretty new to this game, and this issue maybe already covered.
Today I got a loot of an M4A1 and I already had an Mosin with all the perks attached already as main weapon, after some consideration I decided to keep the M4A1 and wanted to empty the Mosin for ammo in my ammo box. Did so with Mosin in hand (Picked up feature, as my M4A1 was in inventory as main weapon) and when I dropped the Mosin it produced two Mosin on the floor with all the perks as the first one. I successfully unloaded the Mosin and I checked the ammo box and all the 7.62 for the Mosin was there as expected.
I see this bug as something too look into, because players can easily exploit this in the game, if it could be reproduced. Unfortunately I did not have any time to test this again because I had to leave the game.