I Remembered a Missed Prayer upon Offering the Due One!
In the name of Allah, we praise Him, seek His help and ask for His forgiveness. Whoever Allah guides none can misguide, and whoever He allows to fall astray, none can guide them aright.
If you have prayed the current prayer, then you remember a missed prayer from the same day or a previous day, you should pray it, but do not repeat the current prayer, because the Prophet (peace be upon him) said: “Whoever forgets a prayer, let him offer it as soon as he remembers, for there is no expiation for it other than that. Then he recited: “Establish prayer for My (i.e. Allah’s) remembrance.” (Ta-ha 40: 14) (Al-Bukhari)
For example, if a person offers the `Asr (Afternoon) Prayer and after finishing that prayer they remember they have not prayed the Zuhr (Noon Prayer), then there is no sin upon them. All they have to do is pray the Zuhr after finishing `Asr.
For more information about such ruling, watch this interesting talk with Sheikh Navaid Aziz.
Source: Faithiq Youtube Channel.