We assume that keriat shema in the evening has to be recited after tzet ha-kokhavim. Yet there are many shuls that on motzaei Shabbat they begin maariv like 7 minutes before tzet, in order to finish maariv when Shabbat is ending. But no one has ever said that people need to repeat shema later on Saturday night, Why not? Furthermore, at all the minyanim I daven at, where we say maariv after shekiah, I have never heard the rabbi tell us to repeat the entire shema later that night. Does one need to repeat it?
Thank you for your question.
It is hard to explain why other people are doing what they are doing, however I can try to offer a suggestion. There are different opinions when exactly is tzet hacochavim, there are many opinions (Rabeinu Tam, Geonim, and a number of opinions how long after shkiya it is even according to the Geonim). Regarding Shabbos some people are more stringent than they are regarding kriyas shema, and they will not do melacha until a later zman. This might explain what these people are doing, however I can’t say this because it depends how long after shkiya they are davening. Regarding the Rabbi, the best thing is to simply ask him, in a respectful, polite way, if kriyas shema has to be said over again after davening maariv in the shul. If he says “yes”, then you can ask him if a sign should be put up to inform the congregants of this.