Parece simples até o entrevistador perguntar "why?". Abaixo, uma estrutura de resposta que dá conta do follow-up.
Idempotent means repeating an identical request has the same intended effect on server state as making it once. Responses may differ. Deleting an existing resource can return 204, then 404 on a repeat, while remaining idempotent.
GET, HEAD, OPTIONS, TRACE, PUT, and DELETE have idempotent semantics. POST and PATCH are not guaranteed to be idempotent, but an API can make particular operations retry-safe.
GET, HEAD, OPTIONS, and TRACE are safe: the client does not request a state change. Logging is an allowed incidental side effect. Safe and idempotent are different properties.
Source: HTTP method semantics.
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