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general::
Phrase(s): Fool me once, shame on you; fool me twice, shame on me.
Prov. After being tricked once, one should be wary, so that the person cannot trick you again. • Fred: Would you like a can of peanuts? Jane: The last can of peanuts you gave me had a toy snake in it. Fred: This one really is peanuts. Jane: Fool me once, shame on you; fool me twice, shame on me.
McGrawhill's American Idioms And Phrasal Verbs