This has now been implemented as part of the changes described here.
Basically, insteadInstead of being edited into the question body, it'sthe banner is now 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 reopened, it's removed. As a bonus, this displays the target's answer count, and is automatically updated if the linked target's title is edited. This is also generated for the author as soon as someone votes or flags to close, along with a button to immediately close the question, so the author can know right away that there's an existing solution to be found and close it themselves.
Since this can't be unilaterally rolled back by the asker, they must instead edit to explain why the question is not a duplicate. At least, that's the theory, and the guidance that is provided in the notices.