It is narrated from Sayyiduna Anas (may Allah be pleased with him) that Bilal (may Allah be pleased with him) was ordered "to say the adhan in pairs and the iqamah once each, except that 'Qad qamat al-salah' should be said twice."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1948