Commentary for Berakhot 66:27
אמר ר' זירא כל האומר שמע שמע כאומר מודים מודים דמי
R. Zera said : Whoever says Shema', Shema' is like him who says "We give thanks, we give thanks" [and is to be silenced].
Abraham Cohen Footnotes to the English Translation of Masechet Berakhot
This is explained to mean saying the Shema' verse by verse twice. Blau, Das altjudische Zauberwesen, p. 147, suggests that the original reason for the prohibition is to be sought in the practice of repeating a prayer for superstitious or magical purposes.
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