Comment by dragonwriter

Draw a diagram containing a source system, a destination system, and an unreliable communication channel between them. The destination system also has an output with no unreliable communications channel.

Exactly-once delivery means that a message sent from the source system reaches the destination system exactly once, and its result reaches the output channel exactly once as a consequence.

Excactly-once processing means that a message sent from the source system produces the expected output from the destination system once, even though it may be received by the destination system more than once.

(That's a little sloppy because it could use more discussion of the conditions in which it won't be received zero times, and how those are different between exactly-once and at-most-once delivery, but that's mostly beside the point because it isn't part of the distinction between exactly-once delivery and exactly-once processing. And, while definitely technical, they always involved a somewhat idealized view of the destination system, because all communications channels, including those internal to a single device, have some degree of unreliability.)