The Same Surah in Both Rakaahs

Q: Salaah-45: What is the ruling when the same Surah was recited in both Rakaahs?

Answer: Without a good reason or excuse, it is Makrooh Tanzeehi and improper to recite the same Surah in both Rakaahs. There will however be no problem if it was done unintentionally. In Nafl salaah, it will be permissible without any abomination.

A narration of Abu Dawood (Vol.1 Pg.l25) states that Rasulullaah SAW. Once recited surah Zilzaal in both Rakaah of the fajr salaah. Ulema state that this was done for a good reason and to make it clear that it is permissible because it was usually Rasulullaah SAW.'s habit to recite long Surahs in the Fajr salaah. Just as the reason for reciting a short Surah was for a good reason and to make it clear that it is permissible, similarly reciting the same Surah in both Rakaahs was done for the same reason. And Allaah knows best what is most correct.

Fatawa Rahimiyyah vol.2