Working with the OS-level semaphore seems like a pretty easy counterexample. Its the "least idempotent" thing I can think of in modern programming.

https://man7.org/linux/man-pages/man3/sem_post.3.html

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In fact, multithreaded synchronization in general is non-idempotent. You don't want to send a synchronize signal twice, you need to send it exactly as many times as necessary. (Ex: mutex lock / unlock, which is probably the simplest version of a sempahore that you can get. Semaphore of size 1)