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  1. A teacher announces, "There is only one student who failed the course."

Does the teacher’s statement mean anything different from the following version?

  1. "There is one and only one student who failed the course," said the teacher.
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    No; the longer version has an identical meaning but allows the speaker to impart more gravitas. Jun 9, 2020 at 11:18
  • Two words never mean the same thing as one word. Four words never mean the same thing as two words. Never. That is true for any words. More to the point, this is a quote. You cannot change it. You have to write exactly what the teacher actually said.
    – RegDwigнt
    Jun 9, 2020 at 12:03

2 Answers 2

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In ordinary conversation, only one has the same meaning as one and only one. The shorter phrase is used almost every situation.

In mathematical logic, it's often desirable to make a distinction between zero or one and exactly one. In that situation one and only one is used to indicate that the count cannot be less than one or more than one.

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The meaning of both "only one" and "one and only one" is the same. However, "one and only one" adds emphasis to the fact that there is only one, and draws attention to it. For example, the student who is the only one who failed, might feel more ashamed if the teacher uses "one and only one", as the teacher might be perceived as purposely drawing attention to that fact, for whatever reason.

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