Praise be to Allaah.
‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) was asked this
question, and he hesitated, then he replied:
best thing is for him to do the second sajdah, so as not to confuse people
and give them cause to shout “Subhaan
Allaah” [to draw his attention to his mistake] – that is, if he is the imaam. And Allaah knows best.