Two arms w/ bones make edit to one side to effect the other?

This is not starting fresh, its working on a custom model for an already designed model that has bones and everything attached to the completed arms. I want to make a modification to one arm and it effect the other. As far as I can tell it is just grouped as one big mesh w/ all the bones.

I understand that if this was fresh I could just make a mirror or ect and do it like that. I just don’t want to have to duplicate one side of the arm and then attach to all the bones and such.

I am very new to modeling again. I used to make models in Milkshape 3D for half-life back in the day and now I’m working on DOTA 2 models. Ugh.

Thanks for any help.