What is the ruling regarding performing a prayer again if one has already performed it?
Chapter on the Times When Prayer is Prohibited
Al-Mughni
Book of Prayer
Primary text
The first prayer performed is considered the obligatory one (Fard). This view is held by Ali, al-Thawri, Abu Hanifa, Ishaq, and al-Shafi'i in his later opinion (al-Jadid). The evidence rests on authentic Hadith stating that the subsequent prayer will be considered supererogatory (Nafl) for the individual. For instance, the Prophet (peace be upon him) said, "If you come to the prayer and find people praying, pray with them. If you have already prayed, it will be a Nafl for you, and this one is Fard." Furthermore, since the first performance discharges the obligation, the second cannot logically be obligatory, as purity of responsibility is achieved by the first act.
Supporting text
Some authorities, citing the Hadith of Yazid ibn al-Aswad where the Prophet instructed to pray with the congregation and that the first prayer would count as Nafl, suggest that the second prayer taken with the congregation is obligatory upon the person if they prayed with the Imam of the locality. However, this interpretation is overruled by the clearer evidence indicating the second prayer is Nafl.