Tafsir of Al-Isra 17:109

ويخرون لﻼذقان يبكون

And they fall down on their chins (faces) weeping, means, in submission to Allah, may He be glorified, and in expression of their belief and faith in His Book and His Messenger.

ويزيدهم خشوعا

and it increases their humility. means, it increases them in faith and submission. As Allah says:

والذين اهتدوا زادهم هدى وءاتهم تقواهم

While as for those who accept guidance, He increases their guidance and bestows on them their Taqwa. (47:17)

ويخرون

(And they fall down), is a description rather than an action (i.e., this is a further description of their humility as referred to in Ayah 107; it does not imply that they prostrate twice)