Possible Duplicate:
Should be [Closed as duplicate] questions redirect to what they are duplicate of?
The thing with possible duplicate questions is that the "search as you type" feature works better when you have some duplicate questions posed in a different way (otherwise, people will end up asking them again). At least that's how it's done on Quora (the old question just automatically redirects to the new question with a question alias).
This is a follow-up question to Why are some questions closed and why are some deleted instead of closed?