Hello, I am saying Nishmat for 40 days in the merit of finding a shidduch. Is there a problem with saying Yishtabach during shacharis and then again when recite nishmat during the weekday? Thank you.
ב"ה Shalom First and foremost, Hashem should answer your Teffilot and you should meet your predestined Shidduch in the nearest time with Mazal and Beracha. As far as Yishtabach is concerned, since it is Bracha it is only to be said only when preceeded by Baruch She'amar and part of Pesukei d'zimra, otherwise it is a Bracha Levatala. (Shulchan Aruch Orach Chaim 53:2). Therefore, you should not say it when you repeat Nishmat whether on shabbat or a weekday. As far as the segula of saying "Nishmat", I found only in the Ben Ish Chai תולדות שנה ב, סעיף ג, שו"ת מכתם לדוד אורח חיים סימן יג) that saying Nishmat is helpful, if a person is in a time of trouble and makes a neder=vow, after Hashem grants the person the salvation, the person should say Nishmat. All the Best