I think there are several things you can do, and they should all be taken as parts of a whole: They should be implemented together for full effect.
Extend "Vote to Close" voting limits for duplicates. In other words, don't rate limit voting to close duplicates.
Effect: This will solve a common problem that these types of questions have, They've been asked beforeThey've been asked before. It will also allow the question to be closed, merged, and then editors can clean up the question (and we generally do).
Create a flag reason called "Flag for editing"Create a flag reason called "Flag for editing". Add this to the existing flag reasons. I can't be everywhere; but I can guarantee if I saw posts that were flagged for editing I'd give those my attention (they should accumulate like other flags so that I can tell which ones need it most).
Editors like editing questions. I happen to love trying to find clearer ways to help the asker ask his question. It's a fun and interesting challenge for me.
Give out a badge for users that edit questions that were at negative score and then received 5 upvotes after their edit. This is only for the editor that changed the most percentage from the original post, and only the first editor to do so before it accumulated the votes (or items that make it work better).
Incentivise finding duplicatesIncentivise finding duplicates.
If a user has an excessive (3?) posts flagged 6 or more times as his post needing editing, he's rate limited in the next question he can ask. That's the built in time-delay system.