(a) Husayn narrated it to us. (b) A group of hadith scholars narrated it from Abd al-Rahman bin Abi Layla (may Allah have mercy on him). (c) Despite the disagreement in its chain, the hadith is mursal, because Abd al-Rahman bin Abi Layla did not meet Mu'adh or Abdullah bin Zayd, nor did he name who narrated it to him from the two of them or from either of them.
Muhammad bin Ishaq bin Khuzaymah said: Abd al-Rahman bin Abi Layla did not hear from Mu'adh bin Jabal nor from Abdullah bin Zayd bin Abd Rabbih, the one who saw the adhan; so it is not permissible to use a report that is not established as evidence against reports that are established.
Other reports on this subject have been narrated through other routes, all of them weak; I have explained their weakness in al-Khilafiyyat. The best chain narrated for saying the iqamah twice is the hadith of Abd al-Rahman bin Abi Layla, and even if it were authentic, every adhan narrated in pairs came after the dream of Abdullah bin Zayd, so it would take precedence over what is narrated about his dream; and there is also disagreement about how the iqamah was in his dream: the people of Madinah narrate it singly and the people of Kufah twice each. The chain of the Madinans is connected (mawsul) and the chain of the Kufans is mursal; alongside the connected report of the Madinans is the mursal of Sa'id bin al-Musayyib, whose mursal reports are the most authentic among the Successors; then there is the order to say it singly that we have narrated after that, and the practice of the people of the two Harams.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الصلاة / 1978