In a nutshell: If a question is a duplicate of another question, flag or vote to close.
When are two questions considered duplicates?
According to Spolsky we should only close real dupes, and according to Atwood, there are three kinds of dupes: Cut-and-paste, Accidental duplicates, and Borderline duplicates (requiring judgement as applied by the community.)
Questions may be duplicates if they have the same (potential) answers. This includes not only word-for-word duplicates, but also the same idea expressed in different words.
Questions asking about the same aspect of the same concept, but with different examples, may or may not be considered duplicates. It depends how easy it is to figure out one example from the other. If it's only a matter of changing some numerical values or some variable names, they're duplicates. If understanding why the questions are at all related requires a detailed explanation, the questions aren't duplicates, merely related.
Which question is the duplicate?
Usually a recent question will be closed as a duplicate of an older question. But this isn't an absolute rule. The general rule is to keep the question with the best collection of answers, and close the other one as a duplicate. If one of the questions is unanswered, the system won't even let you pick it as a duplicate target.
If one question has great answered but bad wording, and the other has poor or no answers but great wording, edit the badly-worded question.
If in doubt, close the more recent question as a duplicate.
It's a duplicate. What do I do?
If you have the privilege to vote to close, click the “close” button under the question, select “duplicate of...”, and paste a link to the duplicate question.
If you don't, but you have the privilege to comment (this requires 50 reputation), click the “flag” button under the question, select “a duplicate...”, and paste a link to the duplicate question.
If you're the first to signal the duplicate, a comment will automatically appear under the question to mention the duplicate.
Should I edit the question?
Do not edit duplicates to mention that — it'll be done automatically once the question gets closed.
Should I answer it?
No, not if you think it's a duplicate. If you don't think the answers on the duplicate question are good enough, write an answer there.
If you don't think the question is a duplicate, then by all means do answer it.
Should I comment?
A comment is added automatically when someone votes to close as duplicate. There's no need to add another comment unless you have something else to say.
Should I flag for moderator attention?
No — use only the “a duplicate…” flag if you can't vote to close. There are a few cases where you may want to flag:
- Abuse cases: when the same user is reposting the same question again and again, flag and let moderators know. Include links to all relevant questions in your post.
- Merging: moderators can merge duplicate questions, which moves all of the answers to the same question. This only works if the questions have identical or very similar wording. If you think two questions should be merged, check whether the answers as worded would make sense on the other question. If they do, flag the duplicate for moderator attention (whether it's already closed or not), include a link to the other question, and explain why you think the merge would be a good idea.
- Unanswered duplicates: ordinary users cannot close a question as a duplicate of an unanswered one. This gives extra exposure to unanswered questions and reduces the risk that question A is closed as a duplicate of question B even though the particular circumstances of question A allow an answer that wouldn't work for B. If you're convinced that two unanswered questions are duplicates, flag one for moderator attention and explain your reasoning.
Should I downvote?
It depends. Duplicate questions are not necessarily bad; different descriptions of the same problem help future visitors to find the answers they're looking for. However, asking a duplicate question may indicate a lack of adequate preparation prior to asking, which is always grounds for downvoting.
What happens to duplicate questions?
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. After that point, the question remains on the site, but new answers can only be added on the duplicate question.
When the question is displayed, a banner will be included to provide a link to the duplicate question (or questions, if multiple duplicates have been suggested). Furthermore, if the question is unanswered, then anonymous visitors are automatically redirected to the duplicate.
Should duplicates be deleted?
In general, no: most duplicates stay around. Having multiple copies of the same question with different wording is useful as search fodder, because people looking for an answer may use different wording too.
Duplicates that are word-for-word copies or that are so poorly written that they are not useful may be deleted by users with sufficient privilege.
Someone says the question is a duplicate, but I disagree!
Questions may be similar without being exact duplicates:
From the blog:
There could be hundreds of different, related, perfectly valid questions on the same topic. There is no One True Question.
It's rarely this straightforward, however -- usually there are two similar but not-quite-the-same questions, both of which have value for different reasons.
If you think a question is related, but not a duplicate, it's a good idea to edit the question to add a note explaining the difference. You can use wording like
Unlike [What color is the sky on Titan?](link), I'm asking about a planet rotating around a brown dwarf.
If you see the question in the close review queue, you can vote to leave open. If the question is closed and you have the privilege to vote to close, you can vote to reopen it.