What man in the Doctrine and Covenants is said to be a perfect man who looked just like his father - so much so, in fact, that they could only be distinguished by their age?
Seth
He (Seth) was a perfect man, and his likeness was the express likeness of his father, insomuch that he seemed to be like unto his father in all things, and could be distinguished from him only by his age.
Doctrine & Covenants 107:43