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I noticed today that a used had asked the same question both on math.SE and on cs.SE (see https://math.stackexchange.com/questions/181291/which-languages-are-decidable#comment417845_181291 and https://cs.stackexchange.com/questions/3119/is-it-decidable-whether-a-tm-reaches-some-position-on-the-tape). The user indicates on another comment that they do this routinely:

Usually when I post there, I don't get answer for couple of hours,so I post here, get answered very nice and fast, and delete my question from csStackexchange.. [comment below Can weight-restricted versions of monotone 2-SAT be decided in polynomial time? ]

The double-posting seems problematic to me, particularly since the questions are being upvoted on both sites. That indicates to me that people are thinking about them on both sites - and the users are not all the same between the two sites. So someone is likely to spend time on a problem only to find that it has disappeared. I think it would be preferable for the user to, at the very least, indicate on the question that it has been posted elsewhere. But ideally the question would just be on one site, so that all the discussion would happen at the same place. To some extent, when a question is on topic for multiple sites, I think it's better to commit to one site for a while before trying another, even if this means the answer is slightly slower.

I tried voting to close this particular question on math.SE as a duplicate, but the system only allows that for same-site duplicates. I tried asking for it to be migrated to cs.SE, but the system doesn't include that as a suggestion.

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    $\begingroup$ I also think this is very problematic and that we should discourage this behavior $\endgroup$ – Belgi Aug 11 '12 at 11:59
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    $\begingroup$ Official SE-policy is that posting on several sites is strongly discouraged meta.stackexchange.com/questions/64068 in general. I think long waiting time for an answer should be measured in units of weeks, not days or even hours. $\endgroup$ – t.b. Aug 11 '12 at 12:11
  • $\begingroup$ Think about all the people in this site whre this qustion you deleted - some of them might work on this problem and write out a solution only to discover that the question is now deleted. If you did not recive an answer during a two days period then you can offer a bounty on your question. $\endgroup$ – Belgi Aug 11 '12 at 12:13
  • $\begingroup$ @Belgi: This is why I posted two notes with links for warning the people about it (After Carl put it to my attention), but it's not legal whatsoever, so I deleted it from mathematics, but discussing about the points I gain from both sites is just detached. $\endgroup$ – Jozef Aug 11 '12 at 12:17
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    $\begingroup$ @Jozef: you are the only one discussing about reputation points here. Carl wrote that the upvotes signal that some people considered the question interesting and thus may be thinking about them. The points are not the issue, I believe. $\endgroup$ – t.b. Aug 11 '12 at 12:33
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    $\begingroup$ If you post the same question to two sites then you must, as a matter of common courtesy, leave a note on both sites saying so, with a link to the post on the other site. $\endgroup$ – Gerry Myerson Aug 11 '12 at 12:37
  • $\begingroup$ O.K, my apologies. I wasn't aware to the aggravation I may cause to people that want to help me. Thank you for pointing it out. $\endgroup$ – Jozef Aug 11 '12 at 12:37
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    $\begingroup$ See also: Questions posted on Math.SE and other SE sites $\endgroup$ – Martin Sleziak Aug 11 '12 at 12:45
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    $\begingroup$ @Jozef: I said what I would consider long waiting time in reaction to a deleted comment of yours where you said that you had no answer in 15 whole hours, which I think is a ridiculously short amount of time to pass to even mention waiting time. $\endgroup$ – t.b. Aug 11 '12 at 12:53
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    $\begingroup$ @Jozef, "15 hours for a student can be very crucial, btw." - for you, sure. But you should know you're imposing on an answerer's time and effort with your question(s)... do you really truly believe your time is more important than the answerer's? $\endgroup$ – J. M. is a poor mathematician Aug 11 '12 at 14:06
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    $\begingroup$ @Jozef: you are asking for our time to look into, study, and then maybe answer your question. This business of multiple-posting and other things is effectively you saying "I don't care about your time or effort, all I want is to have an answer within ~ 15 hours." Very poor form. $\endgroup$ – J. M. is a poor mathematician Aug 11 '12 at 14:27
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    $\begingroup$ @J.M: No, all I tried to do is to expose the question to more people in order to increase my chances to get an answer, that's all. I never complained about not getting an answer, or not getting answer "in time". since we are speaking, I admit that I prefer to get an answer while I'm studying the question myself, and it's not a bad thing. I used to get answers within 5 minutes top on math subjects, btw. this is part of the game here for you, quicker answers, more points. I don't understand this disingenuous. I learned: no answers is because of less exposure. sometimes I don't get any response $\endgroup$ – Jozef Aug 11 '12 at 14:35
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    $\begingroup$ to questions, any views, and when I bounty it it gets 10 votes, couple of answers, and a lot of views. so we learn that people don't answer to question, since they don't expose them. so you can't blame me for trying that more people will see my question. I am out of this discussion. I have nothing to add here. have a great day. $\endgroup$ – Jozef Aug 11 '12 at 14:37
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    $\begingroup$ @J.m: bounty of 500 points on ODE question and concerning about points? come on.. I just want to learn and don't even know how do I get to this kind of discussion. for you being so nice, I'll look for an answer from you and give you 500 points for it. Shalom. $\endgroup$ – Jozef Aug 11 '12 at 14:45
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    $\begingroup$ @Jozef, the general SE policy is against cross-posting for that reason, i.e. "to expose the question to more people in order to increase my chances to get an answer". $\endgroup$ – Kaveh Aug 13 '12 at 21:59
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Given that everything is connected and there's no good line to draw separating CS from mathematics, I think the only really satisfactory solution is if the SE software acquires support for actual crossposting, where the same question and all its answers can be visible on the two sites simultaneously.

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  • $\begingroup$ I think this is a good feature. This way if the original site chooses to delete the question it's fine; and if the new site chooses to delete the question (before substantial addition were given) it's just de-migrated. I can foresee several problems that might come up in reputation; account linkage; etc. if you think of a way to overcome such problems it could make a fine feature request. $\endgroup$ – Asaf Karagila Aug 13 '12 at 20:05
  • $\begingroup$ Unfortunately, I seem to remember that the SE bigwigs have previously shot down such ideas rather definitively. I'd love to find that I'm wrong about that, though. $\endgroup$ – Henning Makholm Aug 13 '12 at 20:51
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The usual way to handle simultaneous cross posts on different sites is to flag for moderator attention and explain the situation. This is something that can't be easily handled by regular users as it requires some coordination across sites.

The moderators of both sites can then decide which of the questions to keep, and either close the other one or migrate and merge it if both version already got an answer.

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Here is my suggestion:

cross-posting an on-topic question is fine as long as:

  • it is not simultaneous, and
  • the OP has waited a reasonable time (a few days) without receiving a satisfying answer before posting it on another site, and
  • the OP has provided links in both directions between the copies.

The OP should keep all copies up-to-date (e.g. if one copy is clarified based on the comments, the OP should clarify the other one also). Similarly, if the user receives a satisfying answer on the other site the user will post an answer linking to the full answer (or do better by posting a complete answer based on what the user has leaned).

In the case that the cross-posting does not satisfy these conditions moderators will close the question as not-constructive and will explain the policy in a comment stating that the OP should wait a week and if the user does not receive a satisfying answer by then the user can flag the question for moderator attention and ask for the question to be reopened.

The comment can be something like:

It appears that you have crossposted this question simultaneously. While we don't mind a question being reposted, our [site policy] (POLICY_URL) only permits a repost after sufficient time has passed and you have not obtained the desired answer elsewhere. I am closing the question since simultaneous crossposting duplicates effort and fractures discussion. Please wait a few days and then if your question is still not answered request a reopening by flagging the question for moderator attention (after updating your question by summarizing the relevant discussions from other sites and explaining why any posted answers are not satisfying).

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How do we spot this? Mostly these would be encountered by accident. Probably the best course of action is to close the question on the less directly relevant site and place a comment to the poster that he should not engage in this practice.

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