From: Rabbi Moshe Sofer
To: Unknown recipient
If one is uncertain whether he recited Krias Shema and Emes V'Yatziv, must he repeat the berachos of Shema even if Shema is only a rabbinic obligation? The Chasam Sofer rules that one must repeat both Shema and its berachos, since the Sages instituted the full form to avoid denigrating the rabbinic components. A second responsum rules that different prayer nuschaos are equally valid and one should follow ancestral custom.
Source: Shu"t Chasam Sofer OC §15