I think it would be enlightning for all if “you” (the Event Store team) shed some light on the subject of idempotency(*), (optimistic) concurrency (detection), command de-duplication (when used in message-flavored cqrs+es), and what behavior client code can expect in the realm of these subjects. A blog post or documentation seems more appropriate than a response in a forum.
Kudos for pulling this off. Your codebase is a feast for my brain.
(*) yes, I’ve seen this one in the docs.