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?

If you said shema before tzeis hakochavim you should say it again after nightfall.