Neo, is this yours or a pic you found online? Ive seen quite a few doubleheaded RES's. Also, living a few miles from the Lawrence Livermore and Sandia Labs, we have a lot of genetically deformed frog and toad species in our area.
Not sure how rare they are, but I know they are expensive if you want to buy one. I'll see if I can find the site I once found. They had all sorts of strange turtles (Two headed ones, albino ones, eyeless ones) for sale for crazy prices.
Not as rare as you might think... Oddly enough, this mutation generally breeds true. I've personally seen them farmed in south florida and sold in tourist traps. Weird but true.