I did not understand the rav’s answer regarding voluntary prayers on Shabbos.
If a person has a safek whether he davened or not, on Shabbos, it makes sense that he cannot repeat the prayer because possibly he was already yotzi so it would be a bracha l’vatala. So safek brachos l’hakel.
You wrote “Considering that voluntary prayers are not permitted on Shabbos“. If a person has a sofek is a separate question?