> you're insistent on calling them different entities so that you can win an internet argument
No, I'm insistent on calling them different entities because in actual practice they can be, and indeed usually are, different entities. De-duping is usually done in the operating system, and applications usually run in user space.
> if the deduplication isn't atomic with the application's actions, incorrect behavior results
So? What do the application requirements have to do with the question of whether or not exactly-once delivery is possible? The application is a red herring. Why do you keep bringing it up?
If you want to argue that exactly-once delivery is generally undesirable, that is not in dispute. What is in dispute is whether or not it is possible, and the application requirements cannot possibly have any bearing on that.
No, as we're saying repeatedly at this point-- the application itself needs to tolerate multiple delivery, because if the deduplication isn't atomic with the application's actions, incorrect behavior results. Stacking on top of TCP doesn't fix this.