I am trying to recall a book that I read as a child -- late 80s, early 90s.
The main character (teen or pre-teen boy) sent away to a mail-order company, and got back a kit for creating a clone. Not sure, but either he had to send a DNA sample along with his order, or the kit required a DNA sample in order to use. The protagonist and the clone had an intermittent (I think) telepathic link between them. For most of the book, the main character's family didn't know about the clone; but in the end the parents of the main character decided to adopt the clone.
IIRC, the word clone was never actually used to describe the clone; probably because it was less in common usage then. Also, there was some kind of final confrontation at the school building, which the protagonist and the clone were able to resolve because of their telepathic link.
I seem to recall the names of the main character and the clone were Wulf and Steve -- not sure which was which.