Siman 67
Presented here for study, not as a practical ruling. The Shulchan Aruch rules by Sephardi custom, the Rema's glosses by Ashkenazi custom, and the Kitzur follows Ashkenazi custom. For a practical question, ask your rabbi.
א סָפֵק אִם קָרָא קְרִיאַת שְׁמַע, חוֹזֵר וְקוֹרֵא וּמְבָרֵךְ לְפָנֶיהָ וּלְאַחֲרֶיהָ. אֲבָל אִם יוֹדֵעַ שֶׁקְּרָאָהּ, אֶלָּא שֶׁמְּסֻפָּק אִם בֵּרַךְ לְפָנֶיהָ וּלְאַחֲרֶיהָ, אֵינוֹ חוֹזֵר וּמְבָרֵךְ:
1 The Law Regarding One Who Had A Doubt If They Recited the Recitation of the Sh'ma - Containing 1 section If one had a doubt if they recited the Sh'ma, they should go back and recite it [again] and make the blessing before and after it. But if they know that they recited it, but are doubtful if they blessed before and after it, they do not go back and bless.