There’s a girl and she’s tired of going to school and doing chores. She sees an ad in a magazine for an institute that can solve her problem by providing a copy of her that will do all the things she hates. She contacts them and they make her clone. Everyday they switch places and she goes to the institute where she can do what she wants while the clone lives her life. Then one day she decides she wants to go back to her life. She sneaks out the institute and she goes home where she sees the clone interacting with her parents. She tells the clone she’s done with her but the clone says they like me better. Then she realizes there’s a switch to turn her off on the back of her neck — and she realizes her best friend has the same switch.
This is Mildred Ames’s Is There Life on a Plastic Planet? published in 1975.
I’m not sure it’s this one, but I’ll throw it out just in case. Is There Life on a Plastic Planet by Mildred Ames.
(I’m sure that I read the book you’re talking about too, but when trying to recall it, the only title that came to me was the one above. I just remember different things about it.)