Excellent back-of-house insight, as usual!
By the time I got back to discuss.python.org, the timer had expired and I was able to post the body of my final “message” draft. Had I not been stopped in my tracks on the first duplicate, I wouldn’t have posted here. (That somehow reminds me of the ‘snake eating its tail’ meme. 
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I’m afraid I don’t think this can be considered a bug, …
Yes, it does seem to fall somewhere in between. This query is about why the first duplicate in a different platform object type would be blocked in the first place. It seems to be a case of trading a solution for a different problem (albeit a lesser one, so the ‘feature’ aspect of this behavior is a net plus as a guard against nuisance posting and messaging). Nevertheless, it seems incongruent and therefore inapplicable to compare a post body with a message body since they have such different scopes.
…since it seems to be working correctly.
I agree, as long as we define ‘working correctly’ to mean “as intended” and this was a design decision rather than an oversight or unintended behavior, which are both a type of bug. 
I didn’t see it posted in features (or discussed anywhere else) and figured this was an appropriate place to report on existing nonoptimal behavior.
Just as any bug you don’t know how to fix becomes a “feature” by default, any “feature” that gets in the way can be considered a bug.
Waiting for the unique posts mins to expire is even a workaround. 
I didn’t have to wait especially long to post the message text, but it does throw a hard stop into the flow. Maybe I’m the first user to try moving content from message to a topic reply. 