Which SpaceX booster became the first orbital-class rocket stage to fly twice?

Falcon 9 B1021 became the first orbital-class rocket stage to fly twice.

The booster first launched NASA’s CRS-8 cargo mission toward the International Space Station on April 8, 2016. After separating from the upper stage, it performed a controlled descent and landed on the autonomous drone ship Of Course I Still Love You in the Atlantic Ocean.

SpaceX refurbished the booster and flew it again on March 30, 2017, carrying the SES-10 communications satellite. That mission demonstrated that an orbital-class first stage could be recovered, inspected, and relaunched rather than discarded after one flight.

The achievement was an important step toward SpaceX’s reusable-launch strategy. Later Falcon 9 boosters completed many more flights, but B1021 holds the landmark distinction of being the first to make a second orbital mission.