If someone mistakenly began davening the weekday Shmone Esrei (amidah) on Shabbos, and he caught himself in the middle of a bracha, he must complete that bracha before he goes on to daven the shabbos davening. The reason for this is since really one should have been obligated to say the full (weekday) davening even on Shabbos, but the chachamim did not want to burden us, so they made a shorter davening for Shabbos, thus the any bracha of the weekday davening is also for Shabbos and therefore he must complete that bracha and only then change to Shabbos davening.(Brachos 21a,Shulchan aruch OHC 268 and Mishne Brurah 268:2).
Question: If someone accidentally said the entire weekday shmone esrei on Shabbos and then remembered that it was Shabbos, and since he didn't mention Shabbos he must now daven the shmone esrei of shabbos. If in his second time he then also accidentally began the weekday brachos does he need to finish the bracha he's in? We might say that since the only reason one finishes the bracha is because really one should have been obligated to say the full (weekday) davening even on shabbos, but this person has already said the whole weekday davening, and he now needs only the Shabbos brachos, so why should he complete the weekday bracha?