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Which HTTP method is not defined as idempotent, so repeating it can create duplicates?

POST is not defined as idempotent. PUT and DELETE are.

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  1. PUT
  2. POST ✔
  3. DELETE

Why: Idempotent means doing it twice has the same effect as doing it once. Deleting the same item twice still leaves it deleted, but sending the same POST twice can create two orders.

Huh, didn't know that: GET, HEAD, OPTIONS and TRACE are also safe, meaning they are only supposed to read, not change anything.

The dad joke
I clicked Buy twice and got two orders. My wallet learned what POST means.
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References

Answer: POST. Idempotent means doing it twice has the same effect as doing it once. Deleting the same item twice still leaves it deleted, but sending the same POST twice can create two orders.

  1. RFC 9110: HTTP Semantics www.rfc-editor.org/rfc/rfc9110
    Of the request methods defined by this specification, PUT, DELETE, and safe request methods are idempotent.
    Checked 2026-10-10.

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