What is the ruling on someone who prayed Fajr and then prayed it again, with another person praying behind him, knowing that he is frequently assailed by doubt?

It is advised not to dwell on obsessive thoughts and not to repeat a prayer after finishing it unless there is absolute certainty regarding the obligation to repeat it, due to the Prophet's (peace be upon him) saying: "Do not pray a prayer twice in one day." Repeating a prayer without a valid reason is considered a violation of the Prophet's (peace be upon him) prohibition. This second prayer does not count as an obligatory prayer to fulfill one's duty, nor as a supererogatory prayer, because it is performed during a prohibited time. As for the prayer of the congregants behind someone who repeated the prayer out of ignorance of the situation, it is valid, God willing, because they performed what they were commanded to do.