If one is uncertain if he said "HaMelech HaKadosh"

by Rabbi Ya'aqob Menashe

During the Aseret Yimey Teshuva one says HaMelech HaKadosh instead of HaE-l HaKadosh. If one, out of force of habit, erred and said (or is uncertain if he said) HaE-l HaKadosh, but corrected it right away, he has fulfilled his obligation. If, however, he paused longer than the time it takes to say "Shalom 'Alechem Ribbi (Umori)", or started the next Beracha, he must go back to the beginning of Shemoneh Esreh.

If, while the person is davening Shemoneh Esreh, he becomes uncertain whether he said HaMelech HaKadosh or not, we consider that he said "HaE-l HaKadosh", like the rest of the year and must repeat Shemoneh Esreh. However, if before starting Shemoneh Esreh he was conscious of the fact that he had to say "HaMelech HaKadosh", and only after he completed Shemoneh Esreh became doubtful whether he said it correctly or not, he does not need to repeat Shemoneh Esreh.