I think this case has a nuance which may be worth calling out. The receiver may be unavailable, which is one case. But there may be also the case where the receiver is actually still processing one request when a repetition of the same message is received. This is a different case, since it may lead to different problems, e.g. deadlocks on resources or inconsistent state.

Get the Medium app

A button that says 'Download on the App Store', and if clicked it will lead you to the iOS App store
A button that says 'Get it on, Google Play', and if clicked it will lead you to the Google Play store