Yeah that's normal - the outside of the claw is supposed to flake off every so often leaving a sharp tip, the front ones they sharpen up by scratching at things (hopefully the scratching post and not the sofa!) but the back ones don't get the same scratching opportunities, so they often do it with their teeth. Horrible noise it makes when they're doing it, makes my toes curl
I always think that Lucy is pulling her nails out of her foot, the way she goes at it. Much has stopped doing it so I have to trim hers because she starts clicking as she walks and they get stuck in things.