In a nutshell: If a question is a duplicate of another question, flag or vote to close.
Note: There are slightly different guidelines for closing as duplicate on meta sites. See the last section for more info.
When are two questions considered duplicates?
According to Stack Exchange co-founder Joel Spolsky, we should only close real duplicates, and according to co-founder Jeff Atwood, there are three kinds of duplicates: cut-and-pastes, 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 one small part of the question (e.g. in the case of coding sites, 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 the selected target question is unanswered, the system won't even let you pick it as a duplicate target*.
If one question has great answers but bad wording, and the other has poor or no answers but great wording, edit the badly-worded question and close the other one as a duplicate.
If in doubt, close the more recent question as a duplicate.
It's a duplicate. What do I do?
If you're the author of the question, and the question isn't closed yet, click the button to accept the duplicate in the notice at the top, and the Community user will instantly close it. If there are no pending votes or flags to close as duplicate, or you think it's a duplicate of a different question, cast one first, then do the above.
If you have the privilege to vote to close, click the “close” button under the duplicate question, select “Duplicate of...”, and paste a link to the target question. If you hold a gold badge for one of the question's tags (provided you weren't the first one to add a given tag), or you are a moderator, the question will be instantly closed, without needing more close votes.
If you don't, click the “flag” button under the question, select “a duplicate...”, and paste a link to the target question.
If you're the first to signal the duplicate, a comment will automatically appear under the question to mention the duplicate. (Note: if you don't have enough reputation to comment, you won't be able to edit the comment.)
Should I edit the question?
Do not edit duplicates to mention that — a notice will be added automatically once the question gets closed.
If you don't believe the question is a duplicate, and you're the author, edit the post to explain how it's different from the selected target (see below).
Should I answer it?
Generally, no, not if you think it's a duplicate. That said, try to look at the proposed target from the perspective and perceived skill level of the asker - could they use a little extra help understanding how the information in the proposed target pertains to the context of what they're working on? Consider adding a comment or editing the automatic comment to explain how or why the other question likely eluded them or why that question answers theirs.
If the asker really needs more help understanding the target's answers and you can't do this adequately in a comment, then you can add an answer that explains those answers. Think carefully about doing this; don't add answers that just re-iterate the answer(s) to the target question, and certainly don't add an answer that simply links to another answer/question.
If the answers on the proposed target need no introduction, then there's no need to provide one. If you want to contribute a better answer, just write one on the proposed duplicate target instead.
If the duplicate you found has no answers, and you can provide an answer that would work for both questions, only post it on one question - generally, the one that is higher quality or shows more research - and vote or flag the other question as a duplicate.
Should I comment?
A comment is added automatically when someone votes or flags to close as duplicate. (You may need to refresh the page to see this comment if you just voted.) There's no need to add another comment unless you have something else to say.
That said, as stated above, if you believe that the author or others reading the question may not understand how the duplicate target answers the question, you should consider adding a comment to explain how or why it does.
Should I flag for moderator attention?
In general, no — only use the recommend closure flag if you don't have enough reputation to vote to close.
There are a few cases where you may want to flag for moderator attention:
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 flag.
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 perfect sense on the other question. If they do, first follow the above instructions and wait until the post is closed as a duplicate, then once it is, flag the duplicate for moderator attention and explain why you think the merge would be a good idea.
* Unanswered duplicates: Generally, a question cannot be closed as a duplicate of a question with no upvoted or accepted answers, unless:
- the site is a meta site
- the questions were posted by the same user
- a moderator closes the question
On main sites, the main point of closing questions as duplicates is to point users to better answers; closing a question as a duplicate of an unanswered question defeats this purpose. Also, this 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.
Try to remember that beginners are still learning terminology, and even the simplest of answers might be eluding them simply because they did not know what to search for. There could also be language barriers at play.
It's difficult to describe the subtleties that differentiate struggling from clearer patterns of abusing the privilege to ask questions; use your best judgement and try to assume the best in people. But if the questions are unclear and the asker is either unable or unwilling to provide clarity, then they aren't useful additions to the site.
What happens to duplicate questions?
If enough 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 target question. Also, pending votes to close the target question as a duplicate of the now-closed question will be invalidated, preventing a duplicate "circle".
Also, the system will prevent users (including human moderators) from voting to delete the target question, to ensure no question is marked as a duplicate of a deleted question. To delete a question with duplicates, one must delete, reopen, or modify the duplicate links so that no question has the to-be-deleted question as a duplicate target. (Note that abandoned questions that are duplicate targets are still subject to automatic Roomba deletion by the Community user if any of the criteria apply.)
When the question is displayed, a banner will be included to provide a link to the target question (or questions, if multiple duplicates have been suggested). Furthermore, if the question is unanswered, and there is only one target, then anonymous visitors are automatically redirected to the target.
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. On the other hand, 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.
See also the "Should I downvote?" subsection above.
Can I edit what the question is a duplicate of?
This can only be done by someone who can bindingly close the question as a duplicate, either by holding a gold tag badge or by being a moderator.
In the past, duplicate targets were edited into the question body instead of being displayed as an automatic notice, so those notices can be freely edited by users with the ability to edit.
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're the author, edit your question to explain how it differs. If you see the question in the close review queue, you can review to leave open. If the question is closed and you have the privilege to vote to close, you can vote to reopen it. Questions edited within 70 days of getting closed will be added to the reopen queue if they're considered to be a "significant edit" by having the "Submit for review" checkbox being checked. In order to increase the chances the question gets reopened, be sure that any such edit after it gets closed fully addresses why the question isn't a duplicate.
Note: On meta sites, the guidelines for duplicate closures are much broader, and it is common for questions to be closed as duplicates even if they don't necessarily follow the specific guidelines above. See Do duplicate closures work differently on meta sites? below for more info.
For more information, see:
Is there a penalty for asking a duplicate question?
No, there's no automatic penalty if your question gets marked as a duplicate of another. However, try to learn from the experience so that you improve your ability to search, by comparing what you searched for with the title of the duplicate target.
A pattern of duplicate questions in short succession will likely attract downvotes from the community, especially from users who didn't think you did enough research to find the duplicate target, which could ultimately affect your ability to ask questions on the site.
Do duplicate closures work differently on meta sites?
Duplicate closures work the same way on meta sites as they do on main sites. However, the guidelines for closing a question as a duplicate are much broader on meta sites, and allow for closures where main sites do not. For instance, a question can be closed as a duplicate of another question that is merely related. One example of this is a question that is already addressed in a small part of another question (e.g. an faq question): in this case, the former question can be closed as a duplicate of the broader one. Or, a feature request that goes against one of the ideals/principles of Stack Exchange may be closed as a duplicate of a question that explains that principle.
Additionally, there are many cases on meta sites where closing a question as a duplicate of an unanswered question is warranted. For example, if the same bug report or feature request has been filed in the past, but received no answers, it should be closed as a duplicate. To that end, the system will allow you to vote to close a question as a duplicate of an unanswered one, which is not allowed on main sites.
Finally, questions may be closed as duplicates in procedural cases, such as the Community Promotion Ads questions on per-site metas: when a new one is posted, the older one will be immediately closed as a duplicate of the newer one, to direct people to the newer one and prevent non-useful answers to the older one. Another example of this is an official announcement later being followed up by an update, in which case the previous announcement will be closed as a duplicate of the updated one so as to direct further queries to the updated one.