Question:

The imam was reciting the quran, when he reached before the verse of sajdah he forgot it. One of the muqtadis (followers) recited the verse of sajdah, there after the imam repeated it. The question is, will one sajdah tilaawat suffice or are two necessary?

Answer:  حامدا ومصليا ومسلما
In the above scenario only one sajdah is wajib. (شامي 1:514 رسائل الاركان 161)
And Allah ta’aalaa knows best.
Hazrat Mufti Ahmad Khanpuri Saheb (May Allah preserve him)
Mahmoodul Fataawaa 1/466-467