Omg It has never happened to me to have the same weapon twice, only barry's shotgun with other characters is very common, just like some weapons that you do not use are saved for the next level.
#Weopon bug in dtm (dan the Man)
vocal seal
mighty onyx
this is very very common, and also the devs are not going to be fixing it
they've said directly that it's an iconic bug and that if they removed it we would miss it
I know that's kind of dissatisfying but that is what they said
as for how you activate it, here's what I said last time someone reported this: