Can a person who has performed or led a fard (obligatory) prayer lead another congregation in the same prayer?

The imam should not be in a lower position than the person following him in terms of the type of prayer performed. However, the congregation may be in a lower position than the imam. Accordingly, someone performing a voluntary (nafl) prayer cannot lead someone performing a fard (obligatory) prayer; but someone performing a fard prayer can lead someone performing a nafl prayer. (Marghinani, al-Hidaya, 1/58-59; Ibn al-Humam, Fath al-Qadir, 1/371; Ibn Abidin, Radd al-Muhtar, 1/590; Dusuqi, Hashiya, 1/339; Buhuti, al-Rawd, 134)

The Prophet (peace be upon him) said: “An obligatory prayer cannot be performed twice in one day.” (Darqutni, al-Sunan, 2/285 [1544]; Ibn Abi Shayba, al-Musannaf, 2/78 [6675]) Since the prayer of a given time cannot be performed twice, the second prayer will be considered voluntary (nafl). In this case, the imam would be in a lower position than the congregation, and thus his leadership would not be valid.

According to the preferred view in the Shafi‘i and Hanbali schools, a person performing a fard prayer may follow someone performing a nafl prayer. According to these opinions, someone who has already performed the obligatory prayer of a given time may lead others in the same prayer. The prayer performed by the imam would be considered nafl for him, while it would be fard for the congregation. (Mawardi, al-Hawi, 2/316; Ibn Qudama, al-Mughni, 2/166)

Source: T.C. Diyanet İşleri Başkanlığı, Din İşleri Yüksek Kurulu