“Verily, your Lord knows that you do stand (to pray at night) a little less than two thirds of the night, or half the night, or a third of the night, and also a party of those with you. And Allaah measures the night and the day. He knows that you are unable to pray the whole night, so He has turned to you (in mercy). So, recite you of the Qur’aan as much as may be easy for you”
[al-Muzzammil 73:20]
Abu Dawood narrated (816) from Mu’aadh ibn ‘Abd-Allaah al-Juhani that a man from Juhaynah told him that he heard the Prophet (peace and blessings of Allaah be upon him) reciting Idha zulzilat al-ard [al-Zalzalah 99] in both rak’ahs of Fajr prayer and (he said) I do not know whether the Messenger of Allaah (peace and blessings of Allaah be upon him) forgot or he recited that deliberately. This report was classed as hasan by al-Albaani in Saheeh Abi Dawood.
‘Abd al-‘Azeem Abaadi said:
The Sahaabah were uncertain as to whether the Prophet (peace and blessings of Allaah be upon him) repeated the soorah because he forgot, as it was his habit to recite a different soorah in the second rak’ah to what he had recited in the first, so that was not prescribed for his ummah, or he did it deliberately to show that it is permissible. So it is uncertain as to whether it is permissible to repeat a soorah or not. If it is not clear whether it is permissible or not, then it is more appropriate to interpret what the Prophet (peace and blessings of Allaah be upon him) did as meaning that it is permissible, because the basic principle concerning his actions is that they were done to show what is prescribed, and the idea that he forgot goes against that principle.
‘Awn al-Ma’bood, 3/23
Indeed, there is nothing wrong with repeating a soorah or a verse within the same rak’ah.
Al-Nasaa’i (1010) and Ibn Maajah (1350) narrated that Abu Dharr (may Allaah be pleased with him) said: The Prophet (peace and blessings of Allaah be upon him) stood reciting a verse and repeating it until morning came. That verse was: “If You punish them, they are Your slaves, and if You forgive them, verily, You, only You, are the All‑Mighty, the All‑Wise” [al-Maa’idah 5:118]. Classed as hasan by al-Albaani in Saheeh al-Nasaa’i.
Al-Bukhaari (5014) narrated from Abu Sa’eed al-Khudri that a man heard another man reciting “Qul Huwa Allaahu Ahad (Say (O Muhammad): He is Allaah, (the) One)” [al-Ikhlaas 114] and repeating it. The next day he came to the Messenger of Allaah (peace and blessings of Allaah be upon him) and told him about that, and it was as if the man did not think much of that. The Messenger of Allaah (peace and blessings of Allaah be upon him) said: “By the One in Whose hand is my soul, it is equivalent to one-third of the Qur’aan.”
According to another report: A man stood at the time of the Prophet (peace and blessings of Allaah be upon him) reciting “Qul Huwa Allaahu Ahad (Say (O Muhammad): He is Allaah, (the) One)” [al-Ikhlaas 114] at the end of the night, and he did not do more than that, but the Messenger (peace and blessings of Allaah be upon him) approved of his repeating the same soorah.
Al-Haafiz said:
The one who recited it was Qataadah ibn al-Nu’maan. Ahmad narrated via Abu’l-Haytham that Abu Sa’eed said: Qataadah ibn al-Nu’maan stayed up for the whole night, reciting “Qul Huwa Allaahu Ahad (Say (O Muhammad): He is Allaah, (the) One)” [al-Ikhlaas 114] and he did not recite anything else…. And al-Daaraqutni narrated another version of this hadeeth via Ishaaq ibn al-Tabbaa’ from Maalik, in which it says: “I had a neighbour who would spend the night in prayer and would not recite anything other than “Qul Huwa Allaahu Ahad (Say (O Muhammad): He is Allaah, (the) One)” [al-Ikhlaas 114]. End quote.
We ask Allaah to enable you to memorize His Book, and to act upon it.
And Allaah knows best.