Why are some questions marked as duplicate?
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 when a question is closed as duplicate, the title is appended with "[Duplicate]" rather than "[Closed]".
- Moderators and anyone with 3000 reputation may vote to close a question as a duplicate by clicking the "close" link and entering in the URL or title of the question they believe it to be a duplicate of.
- It takes 5 close votes to reach the closing threshold. (A question can be closed as duplicate with a single vote from a moderator or 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.
A vote to close as duplicate will automatically post a comment with the duplicate link if no one else had voted for that particular duplicate. The user who cast the vote is the owner of the comment and can edit or delete it like any other comment. The original question generally must have an answer; questions may only be marked as duplicates of unanswered questions on meta sites, when the questions share the same author, or when closed by a moderator.
Once the question is closed as a duplicate, these comments are deleted and the duplicate information is automatically edited into the question itself. Some duplicate questions may eventually be deleted, but often they are left as a signpost pointing people towards the canonical answer to that question.