The fundamental goal of closing duplicate questions is to help people find the right answer by getting all of those answers in one place. This does not mean that every duplicate will immediately be closed; we love (some) dupes. There are many ways to ask the same question, and a user might not be able to find the answer if they're asking it a different way.
Marking a question as duplicate is part of the question-closing procedure, except that when a question is closed as duplicate, the title is appended with "[duplicate]" rather than "[closed]".
- Moderators and anyone with 500 reputation may vote to close a question as a duplicate. They can do so by clicking the "Close" link below the question text, selecting "Duplicate" in the Close dialog box, and entering the URL or title of the question they believe it to be a duplicate of into the search bar within the dialog box.
- It takes 3 close votes to reach the threshold for the question to be closed. (A question can also be closed as duplicate with a single vote from a moderator or from a user who has earned a gold tag badge in one of that question's current tags, unless that user participated in editing the tags.)
- You may cast up to 24 close votes per day.
If a user votes to close a question as a duplicate, a comment with the duplicate link will be automatically posted on that question (if no one else has already voted for that particular duplicate). The user who cast that vote is the owner of the comment, and can edit or delete it like any other comment. In general, the original question must have an answer; questions may only be marked as duplicates of unanswered questions (a) on meta sites, (b) when the questions share the same author, or (c) when closed by a moderator.
Once the question is closed as a duplicate, these comments are deleted, and a link to the duplicate is automatically included in a banner above the question text. If the question closed as a duplicate has no answers, then anonymous users will be automatically redirected to the question it is marked as a duplicate of. Some duplicate questions may eventually be deleted, but often they are left as a signpost pointing people towards the canonical answer to that question.
If a question has been closed as a duplicate, moderators and gold tag-badge holders can then edit the duplicate link(s) for that question to replace, add, remove, or rearrange the list of duplicates. (Such changes to this list are not applied until the "Save" button is clicked.) If a question closed as a duplicate has multiple duplicate links, anonymous users will not be automatically redirected from the closed question, even if it is unanswered.
If you see a question closed as a duplicate but don't agree that it truly is a duplicate, edit it to highlight the differences, then try to get it reopened by casting a reopen vote.