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expressions (phrases, clauses, etc.)
teaching your grandmother to suck eggs, teaching something to someone who knows more than you, preaching to the choir, lecturing to the Buddha
idiomatic expression-
釈迦に説法とは存じますが、医者も禁煙されたほうがよろしいのではないでしょうか。
I'm sure you don't need me to tell you this, but wouldn't it be a good idea to stop smoking?
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