I'll say it again: Pamela Sargent is the most underrated writer in the world. Notice I did not say "science fiction writer" because she visits other genres. (Her biography / novel of Ghengis Khan was incredible.) I first encountered Pamela Sargent in the mesmerizing "Shore of Women" then went on to read the Venus Series and all her previous works.
This is one of her earliest but all the elements are present: The interesting story, the time (not too distant future), the interplay among characters separating the wheat from the chaff, the logical progression of current trends. The subject is particularly timely today with all the wild stories over cloning. (Idential twins are clones - should they be allowed?) The twins are cloned for a reason and without giving away the plot it involves reactions to them and the exploration of space for habitation.