What's the right course of action for cross-site duplicates?

There are two flavors:

  1. A question is posted which belongs on another site and ends up already having been asked there. I can't find an example right now, but it's bound to happen soon if it hasn't already.

  2. The same question is posted in two places at the same time. Take this SO question for example, which is a duplicate of this SF question, posted within a half hour of each other by the same user.

Possible actions:

  • Move it to the proper site and close it once it gets there as a duplicate.
  • Provide a way to close it right away as a cross-site duplicate, without moving it first.
  • Flag it and let a moderator merge them.
  • Close it as "no longer relevant" or something else.

Additionally, flavor 2 is a bit of a special case; perhaps the question really spans both sites in terms of content and intended audience. Should multiple-postings in this case be allowed? If so, should they somehow be linked to each other? If not, how do we avoid a migration war?


6 Answers 6


If it is clearly a question that does belong on another site, migrate it and then treat it as duplicate.

If it is a borderline question, let it live in both cultures, as the answeres will be different, programmers answering for programmers, and admins answering for admins.

Yes, it is important to keep the sites focused, but frankly, I am a programmer and will ask questions in SO mainly. So if I have a borderline question I will ask it in SO, not researching SF or SU upfront.

Linking the questions in a comment or the text will be of help and might point me to some additional answers. But I might not understand them, as the answerer expects too much from me as knowledge of the other community goes.

  • 4
    I also like the "crossover questions" feature that was recently suggested. Jan 18, 2014 at 6:13
  • 8
    With the growing number of SEs I guess that the number of questions that are ontopic on more than one SE is growing rapidly (wordpress.SE having a very big overlap with stackoverflow.SE, physics.SE with astronomy.SE, ...). This answer should also provide a recommendation for these (open, close, migrate).
    – Trilarion
    Jul 21, 2014 at 15:44
  • I would like to point out that a great deal of time has passed (and a great many more stacks created), which might obsolete the basic premise of this answer. Please see my answer to this question for an example.
    – JBH
    Mar 30, 2019 at 0:47

You can't assume that all users will be on all sites. Each site has to stand on its own merits and you have to treat each site as a separate community.

If a question is inappropriate on one site, users now have alternate places to go with it. It's a nice convenience if moderators choose to transfer it for them. If it turns out to be a duplicate, let the other community handle it with the means already in place. If the first moderator happens to spot it as a cross-site duplicate, it can simply be closed and the original poster can decide how to rework the question if they want to try it on another site.

In the edge case where the question is appropriate on more than one site, leave it on both sites and let the users of each community benefit from the information.

  • 18
    There should be a feature for sharing a question between sites for your last edge case. I imagine it would be a product and architectural challenge to implement, and might not be urgent, but eventually.
    – ripper234
    Feb 23, 2012 at 9:24
  • 6
    With the growing number of stackexchanges isn't there the risk that more and more overlap (Mathematics, Computer Vision, Computer Science, MathOverflow, ...) will lead to more and more x-site-duplicates that are ontopic on several SEs. Should we really leave them everywhere?
    – Trilarion
    Jul 21, 2014 at 15:46
  • @ripper234 There is a feature request for crossover questions, which is exactly what you're suggesting. Apr 18, 2018 at 16:57

I just closed a question that was a duplicate of a question of Stack Overflow. I closed it as being too localized and linked to the SO question at the top of the question.

Now I could have:

  • simply copy-pasted the answer from there and garner free rep for it
  • migrated it to Stack Overflow to have it closed there (Shog9 will kill me)
  • ignore the question and keep it on Super User as an additional source of information

However the Stack Overflow version has several answers and an accepted answer, so I don't expect Super User to really add new value. Anyone interested can find his answer by following the link, which is consistent with how it works all over the site.

But also looking forward at potential new members of the SO-family it will be increasingly more useful for users to point to OTHER SO-sites for their answers in a consistent matter. Especially for topics like web-app or iPhone/iPad questions on Super User having the ability to link to a future site would help a lot!

  • i'd've migrated it, with a comment linking to the existing dupe, and flagged the newly-migrated question for a SO mod to close it as duplicate. not sure i'm crazy about the duplicate link you added -- under the principle of least surprise, it should at least show some indication that the link leads offsite. (i've edited to fix, but this isn't normally done so we may be establishing a precedent here...) Apr 7, 2010 at 22:06
  • @quack hmm, I am not sure these are really duplicates, in fact I feel strongly that they aren't. Are they related? Sure. But duplicate, no. Anyway, I reopened and edited out the dupe link in the body -- it's appropriate to leave a related link in the comments though. Apr 8, 2010 at 3:14
  • @Jeff: ACK!... that'll teach me to leap before looking. you're right, those are similar/related but not duplicates. i must admit i don't think i followed the SO link earlier, just assumed Ivo was linking to a crosspost -- ie a literal, exact duplicate. Apr 8, 2010 at 4:44
  • i assumed something like these questions: superuser.com/questions/87822/configuring-linux-network .. serverfault.com/questions/97460/change-linux-network-setup .. that are exact duplicates. which, since the SF question has an accepted answer, i'm about to migrate... Apr 8, 2010 at 4:46
  • 2
    If we can't do cross site duplicates, a rep factory is created, and double work is done for nothing. Many border-line questions have precise and identical answers independent of where they were asked. Dec 17, 2013 at 16:41

I don't think it's any different to duplicates on one site. First step is migration, and then let that community discover/handle the duplicate. When we come across these on SO, we vote to close one based upon its activity, date, structure, etc. I think the same course of actions would be sufficient for cross-site dupes.


I'm from Worldbuilding.SE, and we could use the ability to close as a duplicate of another site.

It may be unusual for some sites. It may even be unusual for most sites. But it's common to have questions asked at Worldbuilding.SE that have been adequately (even marvelously) answered on another site. There's so much overlap between us and other sites (where the most significant difference is context) that we've had to adopt a real-world question policy.

It would be very beneficial to have the ability to close a question as a duplicate found on another site. The very same mechanism can be used, it simply would allow us to look at more than questions from our own site.

  • It minimizes the duplication over many sites.

  • It cross-promotes sites. People may not realize (and regularly don't) that if they strip the "I'm asking about my fictional world..." part out of their question, it can be asked on another site — and possibly even get a better answer. People would see the many sites that touch on the worldbuilding process.1

Note that the original question to which this is an answering post considers a wider range of issues than I have in my answer. Sometimes a question should be migrated because "it's off-topic here." I'm specifically addressing the issue of, "it's on-topic here, but this question has already been asked over there."

Recommendation: enhance the close-as-duplicate process to permit questions from all stacks, not just the current stack in question. If necessary, set a unique rep limit for this ability. I.E., at rep level #1 you can VTC as a duplicate from questions on your home stack. At rep level #2 you can VTC as a duplicate from any stack.

1The astute observer may then ask, "why do we even have a worldbuilding.SE?" I answer, "because there isn't a place where people can ask about how to develop a consistent magic system, or an alternative physics system, or even a place where people can ask about how to create an anatomically correct Orc, elsewhere in the SE universe."


For questions that are on-topic on both stacks, not asked by the same person, and already have a good answer on the other stack:

Link to the other question with a community wiki answer.

Also, it could be good to include the full text of the post in block quotes (using >) in case it gets deleted, and linking to the other author's profile on the other stack.


  1. Allows the question asker to get the information they are looking for
  2. Has no risk of receiving reputation for another's work
    • Prevents any worries of plagiarism

This is the best compromise solution.

  • Well, as long as you quote and reference properly, I don't really see a problem with not making this community wiki. Afterall you usually get reputation for quoting and summarizing other sources from the internet anyway, so why not another SE answer. If the answer's good give that guy his rep, CW is IMHO counter to the workings of SE in those case. May 6, 2015 at 15:10

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