This has now been implemented as part of the changes [described here](https://meta.stackexchange.com/questions/166707/changes-to-close-as-duplicate-part-deux). 

There are a few [kinks][1] to be worked out, but the basic behavior is that rather than editing in the banner, it's generated as-needed and displayed separately above the question (in a fashion that appears more or less the same as the edit would've). If the question is re-opened, it's removed. As a bonus, this is generated as soon as someone votes to close - so the author can know right away that there's an existing solution to be found (assuming he didn't notice the automatically-generated comment that is. 

Since this can't be unilaterally rolled back by the asker, he must instead edit to explain why his question is not a duplicate. At least, that's the theory, and the guidance that should be provided to him.


  [1]: https://meta.stackexchange.com/questions/166427/automatic-insertion-of-dupe-banner-after-just-one-close-vote-is-vulnerable-to-ab