Other example to consider: duplicating a dead/badly asked question
If you have someone coming and asking a question, but not putting effort in it, not providing feedback or details, this question will most likely get downvoted in the end, and forgotten.
Now what if a new user comes, and has the same problem? He searches for existing questions on the site, and finds a very similar one. However, he doesn't own the question, he can't add details to it, all he could do is post a new answer (leading to the many "i have the same problem" answers, which are sometime useful, with new details, but which shouldn't be answers).
In this case, I think he should ask it as a new question, giving his own details. This new question should contain information about the duplicate (to prevent fast people from closing it after a fast search), something like:
I have searched for duplicate, and
found this question (link here).
However, this question is abandoned,
mostly because the author didn't
provide any feedback on his problem.
In this case, the original question should be the one closed for duplicate, and point to this new one. Because we can't let someone "ruin" a problem and prevent others, more motivated, to find a solution.
This particular case of duplicate would help improving the quality of answers on the sites.