A user helpfully flagged my post as a duplicate. I marked it 'helpful'.
Once I did that, the post showed the 'duplicate banner', even though the post itself is still open (never been closed). It was also within the 5 minute window, so there's no revision history. I promise I didn't edit that banner into the post.
If I view the post as a moderator, I see the banner. If I open a new browser in 'incognito' mode, there is no banner.