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Phrase(s): One good turn deserves another.
Prov. If someone does you a favor, you should do a favor for that person in return. • Jill: Thanks for the ride. Jane: It’s the least I can do after you helped me wash the car last week. One good turn deserves another. • Child: I don’t want to help Grandma go shopping. Father: But she helped you with your homework yesterday. And one good turn deserves another.
McGrawhill's American Idioms And Phrasal Verbs
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A favor should be returned in kind, as in I'll give you a ride next time? one good turn deserves another. This maxim was first recorded about 1400, and the converse, One bad turn deserves another, about 1500.
American Heritage Idioms