If both questions have good content, they should be Merged rather than closed.
Otherwise, you should vote to close the one that's worse. Which can be defined in a few different ways. I'll try to put them in ascending order of importance (this is my opinion, nothing more):
- Has poor quality answers (one clinches the deal, as far as I'm concerned)
- Is less general, less canonical
- Has fewer or no answers
- Is older, or concerning an outdated version of some software
- The OP is inactive, the question isn't edited
- Has fewer views (that's "other")
Most of the time, it'll be obvious to you which one deserves to stay. And hopefully, as we discourage answering duplicates, the newer question won't have better answers by the time you see it.