324. Davening the Weekday Shemoneh Esrei on Shabbos

Tefillah u’Birkas Kohanim 10:6

If someone is in doubt as to whether or not he davened, he does not daven again unless he intends the second davening as a voluntary prayer. This is because one may recite voluntary prayers throughout the day. If one is davening Shemoneh Esrei and he realizes that he already davened, he should stop right away, even in the middle of a bracha. If this happened during maariv, he need not stop because he never had in mind that this Shemoneh Esrei was an obligation.

Tefillah u’Birkas Kohanim 10:7

If a person made a mistake and recited the weekday Shemoneh Esrei on Shabbos, he does not fulfill his obligation. If he realized his mistake while still davening, he should finish the bracha he is in and then continue with the Shabbos text. This is only the case for maariv, shacharis and mincha; if it happens in musaf, one should stop davening even in the middle of a bracha. Similarly, if one completed reciting the weekday Shemoneh Esrei intending it to be his his musaf prayer, he must daven musaf again. This is true on Shabbos, yom tov, and Rosh Chodesh.