We have identical twins.
Let's say you had four, all identical, split into two groups ran in parallel simulations. Two in the same womb then split at birth, the other two split at conception implanted in two separate mothers. Of the two borne to separate mothers, one mother is heavily stressed, chronically exposes the fetus to cortisol, while the other is very stable.
When looking at outcomes between these pairs, how do you account for the difference in nature vs nurture when almost all of our twin studies rely on a shared womb, shared first 9 months of nurture?
So it's one of those things that depends on how close you are looking.
https://www.livescience.com/24694-identical-twins-not-identi...
paper (PDF) : https://www.researchgate.net/profile/Constantin_Polychronako...
I think the question stands: what is it about twin studies that is insufficient and needs to be replaced with ethically questionable techniques like cloning?
For those who aren't aware: cloning of vertebrates isn't remotely mature technology. It remains routine for such an organism to be born with significant developmental flaws. Now tell me how you're going to get consent from such an individual pre-conception.
The problem with twin studies is that they are not repeatable, and the sample of just two is too small to make any strong conclusions. It's like saying we do not need particle accelerators, because there are high energy particles coming from space.