Background: having duplicates around is not bad. It is even useful, e.g. easier to find the original due to the additional key words, etc.
That said, I see many users delete their question shortly after it's being closed as duplicate, e.g. latest example:
In this example, having it stick around would, for example, have "send images in mobile" find the original request, where now it does not.
So, my suggestion is as follows: when a user click the "delete" link on their question, and it is:
- Closed as duplicate
- Non negative score
Then a confirmation dialog will appear advising the user to not delete, explaining why. There will be a button "Delete Anyway" of course, but I want to believe this will prevent quite a few deletions of possibly useful contents.