In the Adhan of Bilal, the phrases were two by two, and in his Iqamah they were said once.
Sunan Ibn Majah Book 3, Hadith 26
← Previous Hadith "Bilal was commanded to say the phrases of the Adhan twice and the phrases of the Iqamah once."

Sunan Ibn Majah Book 3, Hadith 25 Next Hadith → "I saw Bilal calling the Adhan in fron of Allah's Messenger, (saying the phrases) two by two, and saying each phrase once in the Iqamah."

Sunan Ibn Majah Book 3, Hadith 27