How about another 'type' of delete, the dupe-delete?
After a question is deleted that is closed as a dupe, then have it queue up for the moderators so they can merge it with the original duplicate. This will help preserve any new useful answers.
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On topic duplicates shouldn't necessarily be deleted - just closed.
If the question is asked in a different form then:
a) the poster didn't find the original as they were looking for it using the "wrong" terms.
b) someone else will come along and look for the question in this form as well and if it's been deleted only ask it again.
c) having the same question asked several ways can only help the Google Juice. As long as they all point to the same "master" question then the knowledge is still in a centralised location.
Of course off topic duplicates should be deleted, well technically they won't be duplicates as the original will have been deleted as well.