What is the legal ruling for someone who doubted completing the final Tashahhud in Maghrib prayer, so they repeated the prayer, then remembered the Hadith: "Prayer is not to be repeated twice in one day"? And is it permissible to repeat the prayer again for a legitimate reason?

Doubt after finishing the prayer is not to be heeded, and the prayer is valid. Repeating the prayer twice is disliked if it is without a legitimate legal reason, because that would assign an unlegislated, specific number to the prescribed prayer. However, if there is a legitimate legal reason, such as praying with the congregation after having prayed individually, then repeating it is recommended and considered a supererogatory prayer.