Actually, it might be possible for a Final Aeon to be acquired multiple times, if the circumstances were right. The limitations on the Final Aeons are as follows:

When the Final Aeon is summoned, it kills the summoner (thus Anima cannot be a Final Aeon, because Seymour survives summoning it and dies later).

After the summoner dies the Final Aeon does not vanish, it remains behind to be possessed by Yu Yevon. The fayth is constantly being used for a summoning (that summoning being Sin), so another summoner cannot call it again (the same Aeon cannot be called more than once at the same time).