Urbit claims, like you, exactly-once messaging is impossible: https://news.ycombinator.com/item?id=14666015

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.