Right. If your definition of "delivered" is "showed a message to the user", then exactly once is a fairly trivial goal to attain. If it's defined "data over t

Right. If your definition of "delivered" is "showed a message to the user", then exactly once is a fairly trivial goal to attain. If it's defined "data over the wire", then it's nearly impossible to avoid some form of ACK/RETRY mechanism on real networks, which means that messages will need to sometimes have to be 'delivered' many times to ensure a single display to the user.