I like to scratch my own head. So I think it is OK if your cat wants to pet himself. Mabey he just has an itch. Or perhaps he is hinting that you should pet him more. Persi like to lay on his back on those cardboard corrugated scratchers and scratch his own back. All this sounds very reasonable to me!
Cosmo too... and he's got a particular fetish with feet and shoes. If I'm sitting in a chair with my legs crossed he'll rub and "pet himself" all over the foot that is slightly suspended... and he'll go nuts doing it when I'm wearing converse sneakers. It's the funniest thing. He's even gone as far as to grab my foot with his two front paws to rub his face harder on the sneaker!