Question Summary:
Mistake In Sunnah Salāh

Question Detail:

If I forget and recite the same surah in the third or fourth rakah which I read in the second rakah. What should I do? Do I have to repeat salaah? 

Answer :

In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
Your salāh is valid. Qadhā is not necessary when reciting the same surah in two or more rakāhs.[1]
And Allah Ta’āla Knows Best
AbdulMannan Nizami
Student Darul Iftaa
Chicago, IL, USA
Checked and Approved by,
Mufti Ebrahim Desai.
www.daruliftaa.net