https://www.nasa.gov/feature/nasa-twins-study-investigators-to-release-integrated-paper-in-2018
Scott Kelly spent one year on ISS. His identical twin, Mark, did not. "The Kelly twins are the only identical twin astronauts in history."
This study did a thorough biological comparison of the two.
Significantly:
There are some fairly prosaic theories for these differences. Still, a very interesting and unique study, with possible far-reaching ramifications.