Once or twice per book and I probably wouldn't even notice it!
With the book I read a few days ago I was lucky to get 10 pages without a double page. At one stage it happened 3 page turns in a row. I must say I was thinking very bad thoughts about my Touch at that point.
I turned the Touch off and back on, I resynced my books, I cleaned the screen, I washed my hands, I did everything I could think of to try and stop it.
The only thing that seems to have worked was starting a different book - and I can't begin to imagine why that would help?