What happens when I flag as duplicate? Usually they are closed by others, but sometimes I saw that the flag was helpful but nothing happened to the question. Are duplicate flags shown to some special people?
From help (emphasis mine):
Any post which currently has an active close vote or a close flag will appear in the Close Votes review queue. In this queue, users can either vote to close the question, edit the question, or recommend leaving it open. Edits or a sufficient number of "Leave Open" reviews will remove the question from review and immediately begin aging the close votes. If the question receives another close vote after being kicked out, it will re-enter the queue for further review. Questions with vote to close as a duplicate will also list each possible duplicate in a tab at the top for easy reviewing.
Users with 3k reputation can review close votes in the close votes review queue. So if you flag a post as "should be closed" or "duplicate" then the post gets into this review queue. Here other users can vote whether the post should be closed or left open.
Once you have 3k reputation points, you can vote to close. For 3k+ users, close vote and close flags have the same effect, see here for details: http://meta.math.stackexchange.com/questions/21547/what-is-the-difference-between-voting-to-close-as-a-duplicate-and-flagging-as-a
I'll clarify the reason for "the flag was helpful but nothing happened to the question". A close flag is marked helpful if either of the following happens:
- Someone votes to close for the same reason as you picked
- The question gets closed, for whatever reason.
In the situation 1, it's possible for the flag to be marked helpful even if the question never gets closed.