It's worth remembering that the same applies for person-to-person interactions too -- there's nothing magical about software that makes it somehow more suscepti

It's worth remembering that the same applies for person-to-person interactions too -- there's nothing magical about software that makes it somehow more susceptible to this kind of problem, it's just often easier to resolve with people. Especially if you're close enough for so-called "real time" communication. Exactly-once delivery means solving the Byzantine Generals problem, and since that's not possible, neither is exactly-once delivery. Which isn't to say you can't get a good enough approximation, but you'll still only have an approximation.