I've been studying breeding and its effects on a bitch quite a bit lately because of my interest in breeding someday. To me, the question of breeding back-to-back cycles can't be answered with a simple "yes" or "no".
According to some canine reproductive specialists, breeding a healthy bitch in back-to-back cycles is not necessarily a bad thing. If you google Dr. Robert Van Hutchison you will find some transcripts of seminars that go into that. The uterus of a bitch deteriorates with every heat cycle. Breeding her doesn't create any more deterioration of the uterus than not breeding her would.
Of course, the bitch needs to be healthy, maintaining weight, and be a good mother to start with. Bitches carry their puppies for approximately two months, puppies are generally weaned by six weeks old, and that gives her a couple of months to recuperate before being bred again. Some bitches can do it, some can't.
There are a lot of other things to consider besides the current health of the bitch. How old is she? How easily did she whelp the first time? Did she produce dead puppies? How easily did her milk come in? Did she enjoy the puppies? Was she bred the previous heat cycle too? Is she of a high enough quality that she should even BE bred? What kind of health testing did she have prior to breeding? What about the stud dog? Is he health tested?
If the motivation for breeding again is a quick sale of puppies, then I would say that's absolutely unethical. The current puppies are only four months old - WAY too young to truly assess for quality and health - so there's no way of really knowing if this is an "excellent" breeding.
Melanie and the gang in Alaska