How is victory decided when two civilizations reach Alpha Centauri in the same year?

If they both have the same year of arrival, the one that arrives first wins. Freeciv tracks flight times as decimals, not integers, so a ship with a 15.2-year flight time beats one launched a year later with a 14.4-year flight time.

If both ships have identical arrival times (which pretty much requires identical ships launched the same year), turn order is used as a tiebreaker: the civilization that would take their turn first is the winner.

Victory is evaluated at the start of each year, and there are two things that can preempt a spaceship victory: hitting the turn limit, or a civilization winning through cultural domination (if it's enabled).