Are you sure you have a RES, and not a yellowbellied slider? They are subspecies of each other, and the shell looks the same for both. As for double scutes, it's common enough to find those in the pet trade. Some people say it's from too high a temperature during incubation, which would be why so far I've only seen it in females. I've seen a lot of Myrtle Beach and NYC babies (once they've grown up enough that people don't want them any more) with double and even triple scutes. One turtle I took in had 21 vertebral and plural scutes, instead of the normal 13.
Here are some pictures of yellowbellies.
http://www.turtlefarms.com/specialty_gallery/yellowbelly_normal.jpg
http://www.uga.edu/~srel/govern'sschool10-2-02.htm
As for breeding with a painted, I don't think that would create viable offspring, or at least not offspring that could reproduce because they are different species. Why would you want to create a hybrid anyway?
Katrina