According to Meta.StackOverflow...
Ideally, close the original as the duplicate of the new one.
In this case flag one of the questions for moderator attention explaining why you think the closing was the wrong way round and they should be able to sort it out.
- ChrisF
Most of the time double posts by the same user are caused by back-button edits, so the newer question is the one the OP really wants to stay. Even if the questions come hours or days apart, I'll usually elect to keep the newer one, since it will typically have more/better content.
In the case of duplicates by different users, I do take the dates into account. The older question usually takes precedence, if all other things are equal. The quality of the newer question can definitely sway my decision though.
- Bill the Lizard
Just merge them.
- Jeff Atwood
If the new question is a better question or has better answers, then vote to close the old one as a duplicate of the new one.
You can flag and ask a moderator to merge after closure if they're exactly the same.
If they differ based on the versions of the relevant systems, then they're not really exact duplicates - make sure this is indicated in the questions themselves, and provide cross-links so folks looking for one but finding the other have a path to the correct answers.
- Shog9