I'm afraid this question itself is very likely to be a duplicate, but I haven't been able to find this information by searching Meta.
How exactly does the system decide when to close a question as a duplicate?
I know a question must be closed in order to be a duplicate. Moderators and users with a gold tag star can do this unilaterally. Regular users must put in 5 close votes.
The FAQ page "How should duplicate questions be handled?" says:
If five close-voters agree that the question is a duplicate, or if the asker agrees, or if a sufficiently privileged user votes to close, then the question will be closed as a duplicate.
But in my experience, the question can be marked as a duplicate even some of the close votes are for a different reason. I'm not sure, but I think it just requires a majority (three users) to agree that the question is a duplicate.
I've also seen cases where a question is marked as a duplicate of more than one other question. For example: Which is correct — “a year” or “an year”?
How did this happen? Is it still possible, or is it some relic of an older system for closing as duplicate?
Since it seems people can only vote for a single duplicate, I'd assume this means different users made different votes. Am I right? I'm not sure what the numerical requirements would be for this--does this happen if at least two users vote to close a post as a duplicate of one question, and at least two users vote to close it as a duplicate of another? After looking more closely, I noticed the second duplicate target is marked as a duplicate of the first. So my best guess is that this happens when the target of a duplicate becomes marked as a duplicate of another question.