What is the difference between voting to close as a duplicate and flagging as a duplicate?

Question

I am aware that to flag you only need 15 reputation points. Only users with 3k+ reputation can cast votes.

• If a post is flagged as duplicate, what happens then? In particular, what is different from casting a close vote? (I suppose that in both cases the post enters the review queue. Is the only difference that if the post is flagged, then it gets into the review queue with no close votes, so one more vote to close is needed?)
• For users who already have privilege to vote to close, are there any reasons why they might choose to flag a question as a duplicate rather than vote to close as a duplicate?

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EDIT: I have found this post on meta.SE: Vote to close or flag exact duplicates? Or both?

The recommendation in answers there seems to be that if a user has sufficient reputation, voting to close is preferred to flagging as a duplicate, with some exceptions.

This would answer my second point. But perhaps somebody will have something to add to what's written there. And there still remains the first questions whether there are some important differences between the two things. (With the exception of the obvious difference that a low rep user cannot cast close vote, so if such user flags a post as a duplicate, no close vote will be added.)

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I have to admit that when reviewing close votes I often open the question in a separate window. (To have a look at comments and answers already posted there.) If I see no close votes there, I get suspicious that this might be a review audit. This might influence the way I review the post, especially if it seems to be borderline.

Answer 1

Q1: See the help page:

Any post which currently has an active close vote or a close flag will appear in the Close Votes review queue.

The same review process will occur, the only difference is that the post will enter the review queue with one close vote less. The difference for the user who flagged is that instead of using up a close vote, it will use up a flag, and this will appear in their flagging history (though if the review doesn't end up in closing, the flag will be marked as "disputed", not "declined").

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Q2: If a user has sufficient reputation (i.e. ≥3k on a non beta site) then voting to close (as a duplicate or not) is exactly the same action as flagging to close. The user will use up one of their daily votes (but not one of their flags and it will not affect their flagging history at all), the counter on the question will increase by 1, and the post will enter the review queue if it's not already in it. There is absolutely no difference between flagging and closing here.

It doesn't matter whether you use the close dialog or the flag dialog, so it's hard to make a recommendation here -- using the close dialog is one click less, so I'd recommend that. But in the end it's exactly the same thing.