The current description for Mark As Duplicate is as follows:
This question has been asked before and already has an answer. If those answers do not fully address your question, please ask a new question.
I feel that this may give new users the wrong impression when they double post the same question. Let me give an example.
The above question was similar to a previous question of his where people had pointed out to him that what he was doing is impossible and that refactoring is the only solution. He tried the impossible anyway and posted the above question, to where he got an answer. The answer was well intentioned and informative (even though there was no real solution to the problem) however the OP didn't understand.
Two days later he reposted the same question with a different title pleading for help again.
The first impression here is to close the the double post as an exact Duplicate of the first one, however the text is very much worded to the assumption that somebody else has already asked this question and received one or more answers on it. I will not go into this debate here but if you interested the discussion for this is here: When is a duplicate question not a duplicate?
As the Mark Duplicate description is worded now it may give this user the impression that since his previous questions did not give him the answer he was hoping for then he should continue to post it over and over again.
Perhaps this doesn't seem as obviously unfair and selfish to others who do this as it does to me, I like to presume that when people are breaking the rules that the most common reason is ignorance of the rules, and barring that I will come to the conclusion of selfishness and lack of ethics. Perhaps if we modify the text to say something to the effect of:
The question has been asked before and either already has an answer or is a question that has already been posted before by this user. If the answers in the other question do not fully address your question, then please ask a new question if you have not previously asked this question before.
By being explicit here then nobody has an excuse to hide behind the veil of ignorance.
The other option I see is possibly created two different types of Duplicate close reasons.
Related Discussion: What's the policy on marking questions as duplicates?