Interesting. I'll elaborate slightly before passing this to others:
The bug only works in storage. It seems that inspecting ANY weapon in storage inflates the quantity of the first two weapons.
However, while I can repro on the attached save, I can't repro on my personal save.
Edit: in testing here, this seems to be a UI bug. Leave the storage and come back, and the duped items are gone. How were you able to save with the duped items?