If a person prays Maariv after Plag HaMincha prior to Shekiya is there any Halachic authority that holds he is Yotzei the Mitzva of Kriyas Shema and does not have to repeat it? What if he prays right after Shekiya before Tzais HaKochovim?
Tosfot on Brachot 2a (link) say (quoting Rabbeinu Tam and the Ri) that the essential Kri'at Shma is the one in shul, even though we say Shma while it is still daytime.
Rashi, on the other hand, rules that one must repeat the first section of Shma after dark, and the bedtime Kri'at Shma suffices.
The Raavyah agrees with Rabbeinu Tam (according to the Mordechai, according to the Bach: link), but, generally speaking, the Ge'onim, Rishonim, and Acharonim rule that one must repeat the Shma after nightfall.
can't remember if anyone speaks of this case specifically, but in a general rule ben hashemashot is considered safek if is day or night. kiriat shema is mideoraita so safek deoraita lehumra.