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From all reference I have read, the phrase "exactly-once messaging" does not have a strict technical definition. Neither of your links define or use the exact term "exactly-once messaging". If you can point me to a definition of this specific phrase's standard terminology, I would be grateful. But, I cannot find one.
I don't understand why you address me with such a hostile tone. What have I done to deserve this? You accuse me of playing word games, but this is not my intent. I am just trying to have a conversation about the technical aspects of a system.
But that doesn't mean we can't provide systems where, for all practical purposes, it is a guarantee.
Another way to think about it: SHA1 cannot provide a unique hash for every possible set of content - there are overlaps. This is the pigeonhole principle.
Yet, we have entire systems designed around assuming SHA1 is and will always be a unique hash for a piece of content (Git), and, for all practical purposes, it is a guarantee good enough to work despite that.