I just got a mod for my varmint rifle, and I was wondering if I should put it on, or should I save it for a special version of the varmint rifle. For what I can tell you cannot remove mods once they are installed, and I am worried that maybe there are only a 1 of each per game.

I'm fairly certain you can get mods more than once, I'm pretty sure Ive seen some duplicates on traders. Even if you can't, I don't think you can attach mods to special weapons, or at least I couldn't on Maria.