Nasai

861. Commentary: Imam al-Nasa’i rahimahullah understood from the mentioned narration that Ibn ‘Umar radi Allahu anhuma had already performed the congregational prayer. People were praying individually, or it is possible that there was a second congregation, and then this conversation took place. If this was the situation, then the response and deduction of Ibn ‘Umar radi Allahu anhuma are correct. However, the apparent wording suggests that the congregation was taking place and Ibn ‘Umar radi Allahu anhuma had already prayed alone and was sitting. In this case, his deduction is questionable because it is contrary to an explicit hadith. It seems that the aforementioned ahadith of the Prophet sallallahu alayhi wa sallam were not known to him; otherwise, repeating the prayer is only prohibited when the first prayer was performed in congregation in a complete manner, there is no reason to repeat it, or the intention of obligatory prayer (fard) was made both times. This last interpretation and reconciliation is that of Imam Ahmad and Ishaq ibn Rahwayh rahimahuma Allah, and this is what is meant by the hadith. For further details, see: [ذخیرة العقبٰی ، شرح سنن النسائي : 10؍348]

579. Commentary:
This means that one should not repeat a prayer on one’s own accord without any reason; however, if there is a reason, then it is permissible to do so. For example, if someone has performed the prayer alone and then finds a congregation, or joins someone praying alone as an act of charity, then it is permissible. [حديث۔ 574]
Or if he leads someone in prayer, that is also permissible. [حديث 599]
In these cases, the prayer performed the second time will be considered a voluntary (nafl) prayer for him.