Many Poskim say to avoid T’fillos Nedava altogether nowadays because we can’t always have Kavana properly
For the sake of nedava, you’re right. When it’s to avoid a shailah (such as our case), I’ve seen that we do.
R’ Chaim Kanievsky in Shoneh Halachos does not bring the opinion not to repeat.
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http://hebrewbooks.org/pdfpager.aspx?req=49174&st=&pgnum=411&hilite=