A man from Juhaynah informed him that he heard the Prophet ﷺ recite in the dawn prayer "When the earth is shaken" [Zalzalah 99:1] in both rakahs. I do not know whether the Messenger of Allah ﷺ forgot or recited it that way intentionally.
أن رجلا من جهينة أخبره أنه سمع النبي ﷺ يقرأ في الصبح: {إذا زلزلت الأرض} [سورة الزلزلة] في الركعتين كلتيهما، فلا أدري أنسي رسول الله ﷺ أم قرأ ذلك عمدا.
Abu Dawud narrated it (816) from Ahmad bin Salih, Ibn Wahb narrated to us, Amr informed me, from Ibn Abu Hilal, from Muaz bin Abdullah Juhani, and he mentioned something similar. Its isnad is hasan due to the discussion surrounding Ibn Abu Hilal, who is Saeed bin Abu Hilal Laythi, their freedman, Abu Ala Misri, though some say he was from Madinah, and his teacher Muaz bin Abdullah Juhani, except that both of them are trustworthy narrators. Nawawi said in Khulasa (1226), "Abu Dawud narrated it with a sahih isnad," and Shawkani said in Nil (2/54), "Its narrators are the narrators of the Sahih." I say, Muaz bin Abdullah Juhani was not narrated from by Bukhari and Muslim, rather the compilers of the Sunan narrated from him, as did Bukhari in Khalq Afal Abbad.As for the companion's statement, "I do not know whether the Messenger of Allah ﷺ forgot, or recited that intentionally," the default principle is that an action of the Prophet ﷺ is considered legislated. The companion's uncertainty between forgetfulness and intent is resolved in favor of intent unless evidence proves otherwise, and I have not come across anything prohibiting the repetition of a single surah in the two rak'ahs. That is why Abu Dawud and others titled a chapter, "Chapter on a man repeating a single surah in the two rak'ahs."