https://motherboard.vice.com/en_us/article/d3nzgz/everything-weve-learned-from-nasas-study-of-identical-twin-astronauts

Identical twins Mark and Scott Kelly were selected by NASA to be astronauts in 1996 because the brothers had extensive experience as naval aviators. The twins were solid astronauts—they racked up missions on the Space Shuttle and then the International Space Station (ISS)—but the agency eventually recognized the unique scientific opportunity presented by two genetically identical astronauts.

This idea culminated in the NASA Twins Study, which was part of Scott Kelly’s One Year Mission onboard the ISS. From March 2015 to March 2016, the Kellys underwent the same medical procedures and tests in space and on Earth, with Earthbound Mark acting as the control to spacefaring Scott’s manipulated variable.