It doesn't have to be a new event. It can be the exact same event as the prior runs, leaving those that have done it before with no concerns (as they won't have to worry about being bothered by it again).
It's just how MMOs work. Think of how many times events are repeated each and every day, and nobody bats an eye about it. Every single player here has been (for example) duped by a traitor, captured by the Amalj'aa, and served up on a platter to Ifrit - and then defeated him. All the exact same story, all at different times, from the day the game launches to the day it goes offline forever.
There is absolutely zero difference between that and new players experiencing Lightning's event. Most events in this game are not one-shot. And I am unaware of any rule that states "seasonal" events have to be.
