I had a recolored (green) Blaze 95 with Hunting Scope and ammunition attached stored in a barrel (open lid).
I equipped the gun directly from the barrel onto my back.

I then proceeded to do some other inventory management, shuffling stuff between barrels etc, but then the server restarted.

When I logged back in the Blaze was duplicated, both one on my back and one in the barrel. Same color, all attachments.

I confirmed it was a legitimate duplication and not just a client error by taking off scopes from each individual gun and remounting them. Afterwards I took both guns (on in hands, one on the back) and logged onto another server, keeping both guns.