Abraham Lincoln to Moe Howard; and the 'normals' or 'Zombie Hunters' are often based on pop culture zombie hunters like Sheriff Rick and Shaun of the Dead, or simply mere pop culture icons like Chuck Norris and Crocodile Dundee (neither of whom fought zombies to my knowledge.)
That Figures who Double-Barrel Carol's inspiration is:
Travis Pitts has made the internet rounds for a while now. Double-Barrel Carol is so close to this Velma (while being nothing like any other zombie hunter of pop culture fame) that she simply must have been inspired by this piece. Which makes her even more awesome than without a reference source, if that's at all possible!
Now to find an in scale Scooby Doo figurine!