بسم الله الرحمن الرحيم
(Fatwa: 212/183/SN=03/1437)
There are several Hadiths of the same meaning which imply that seventy angels will be busy in writing the rewards for one thousand days for the person who recites the aforesaid durood. However as far as the question ‘if someone recites it twice then what is it reward’ is concerned we could not find it mentioned anywhere, but it is hoped that Allah may appoint another group of the angels to record the reward. See: Fazail Amal/ vol. 1, p. 43, pub. Farman Book Depot/ chapter: Fazail Durood.
Allah knows Best!
Darul Ifta,
Darul Uloom Deoband