There was a series of books in my school library when I was a kid (1990s, US) about a future where human-like androids were common. The main character was a kid who got his own android double in the first book, and they had low-stakes adventures together.
Details I remember:
- The androids looked exactly like humans, except that they could only walk without bending their knees.
- The protagonist got good at walking like this, and would sometimes get out of trouble by pretending to be his double.
- The plots were vaguely Hardy Boys-like, so I think there was at least one about the protagonist getting kidnapped. There was also at least one about visiting an android factory, which might have been the first one.
- Solar cars had replaced gasoline cars.
- I think it was common in the setting for people to have robot doppelgangers, but I don't remember why they would and it might have just been the protagonist. I think he got his as his birthday gift but I'm not sure.
- I think they were published in the 70s or 80s. The books were definitely a bit old by the time I read them, and I remember the cover design being full of flat colors and stylized fonts like a 70s movie poster. One of the covers might have had a solar car on it, or a road.