לרפואת אבי מורי ר' יצחק יונה בן חנה בתוך שח"י
Let's say you ate cake and grapes. You now have to make ONE Bracha "אחת מעין שלש" which combines על המחיה and על העץ. If you only made an על המחיה then you have to make another bracha of על העץ. It would seemingly be forbidden to mention על המחיה in the second bracha b/c you were already yotzei the first time around. If it was Shabbos and you forgot to mention Shabbos in your על המחיה, should you mention על המחיה in the second bracha together with על העץ and Shabbos? Apparently NOT. Your על המחיה was already covered the first time around, albeit w/o the proper mention of Shabbos. So saying על המחיה the second time would be not only unnecessary but a הפסק.
Now let's say it was Shabbos Rosh Chodesh and you said the WEEKDAY Rosh Chodesh Mussaf? Now you have to repeat Shmoneh Esrei by saying the Shabbos Rosh Chodesh Mussaf ["אתה יצרת"]. But let's say the person repeats Shmone Esrei and said the REGULAR Shabbos Mussaf. So now he said the weekday RC Mussaf and the Shabbos standard Mussaf. Here, both tfillos are not לכתחילה. In the previous cake-grape case, both times he made a proper bracha [besides omitting the הזכרה of Shabbos]. But here, both Mussaf's were faulty and non-לכתחילה. What should he do NOW? On one hand - he said Mussaf [first time] and he said a Shabbos tfilla [second time] so maybe he covered both bases and he is good to go [but maybe first five minutes in the penalty box before going to kiddush and filling his plate with goodies]. Or maybe no. Today, the Mussaf is Shabbos RC - אתה יצרת, and he HAS to say that. As they say in French " גאָרנישט וועט העלפן" - nothing will help.
The resolution? Say Moreinu HaGaon Rav Avraham Genechovski ztz"l; It would depend on a different Machlokes. What is the din if a person says יעלה ויבוא and forgets ותן טל ומטר לברכה. Then he davens again and says ותן טל but forgets יעלה ויבוא? Can we COMBINE the two tfillos and he is yotzei or no, he needs ONE proper tfilla and we don't make, as they say in the modern parlance, "קומבינות", contrived and artificial combinations? The Gur Aryeh [או"ח סי' י"ז] says that we can combine the two. The Mateh Ephraim [תקפ"ב כ"א] says we cannot. Here too, according to the Gur Aryeh, we may combine the two Mussafs but not acc. to the Mateh Ephraim.
