Note that while the symptoms were the same (the dupe notice disappearing), it happened under slightly different circumstances, in that it was a live, active question with other people editing their own answers, rather than opening a new tab to an old, already closed question.
Reconfirmed the bug on this question. The answer got edited, reloading it caused the notice to disappear. I used this search to watch for likely instances of it occurring. Note at the time of writing, that question has 2 reopen votes, but it is/was a duplicate at the time of refreshing the answer.
Seems that it's not just happening on duplicates. I've just witnessed the same behaviour on a question with a Bounty notification banner - reloading a edited answer also causes the bounty notification to disappear.