A. Both are valid according to Sunna, but Hanafis and Hanbalis prefer a double repetition, while Shafi`is and Malikis prefer a triple one. (For Shafi`is see Nawawi's remarks in al-Adhkar, chapter on the dhikr prescribed by the Law in the two `Eids.) There is no specification about it in the hadith, but the triple repetition: allahu akbar allahu akbar allahu akbar was the practice of five of the major Companions, see Shawkani's references in Nayl al-awtar. wallahu a`lam.