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The problem:

Several times a day, something like this happens:

Now, the main problem is that – if it's necessary to post this at all – the comment should rather say, "Your question belongs on Super User, wait for it to be migrated there and don't post again.".

However, you can't get all users to do that. Want another example?

  • This one was posted on Stack Overflow.
  • A SO user tells them that it belongs on Super User.
  • OP says: "I have posted the question on SuperUser as well now. Thank you!" (…)
  • Same problem as above.

This results in a great moderation overhead on the target site. We will have to deal with:

  • Finding the duplicate between several revisions of the same post
  • Accumulating enough duplicate close-votes (this is harder than it seems if you don't have enough active users with the necessary reputation)
  • Merging answers and comments
  • Telling the OP to stop cross-posting
  • Telling the OP that it's not okay to cross-post, even if they were encouraged to do so

The solution:

Therefore – similarly to Block migration if user is suspended/question-blocked at destination – I propose the following filter:

If a migration procedure is triggered, the target site should be checked for already posted questions with exactly same content. If there already is one, block the migration and close the question as off-topic on the initial site.

Alternatively, as per Gilles' answer:

If a migration procedure is triggered, the target site should be checked for already posted questions with exactly same content. If there already is one, migrate and merge those questions.

There are some optimizations that could be implemented to reduce the search costs involved:

  • If there already is an associated user account, check this account first, as it is likely the one being used for cross-posting.
  • If the former is true, do a simple check for titles in this user's questions.
  • Only compare against first revisions on the target site. If a user copy/pastes the content, it doesn't matter how it was changed on the target site (e.g. Super User).
  • Only search for questions posted after the date the original (e.g. Stack Overflow) question was posted. This is valid for the cases I mentioned in my example above and obviously doesn't work when the OP posts to multiple sites simultaneously. Maybe it would be an idea to make it a more fuzzy search.
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Would "same user and same title" be a criteria? –  Paŭlo Ebermann Sep 26 '11 at 16:48
    
@PaŭloEbermann In the simplest case, if there is a matching user, yes. Some users however aren't registered on the other site. This is especially the case for Stack Overflow: If a user posts there because they only know SO, it is very unlikely that they already have an associated user account on the target site. Otherwise they'd probably know it'd be off-topic for SO and on topic there. –  slhck Sep 26 '11 at 16:50
    
A lot of times, some will post on a number of sites first, then the off topic sites would migrate them to the appropriate site that already has the question leaving the destination site with 2 or more exact duplicate questions usually with different answers. Yeah... it's annoying. –  Jeff Mercado Sep 27 '11 at 0:29
    
Related on SU meta: meta.superuser.com/questions/4030/… –  DragonLord the Fiery Apr 29 '12 at 1:03

1 Answer 1

It's not so simple: if the question has had answers on the site that it's migrated out of, it's often good to migrate those. (Not always, I very well know that answers to off-topic questions can be low-quality.)

What I'd prefer (I see this mostly from Unix & Linux, which is in a similar situation but with a lower volume and only moderator-initiated migration) is for duplicate questions to be flagged (like duplicate answers are). For this purpose, tags should be ignored (as different sites may have required the asker to choose different existing tags, or synonyms may be different). Perhaps even small variations in content should be ignored.

In fact, automatic flagging of near-identical questions would be useful even outside the context of migrations. This is difficult to do efficienty; at least flagging identical bodies would be a good start.

It's true that it would be good to outright reject identical bodies, but in the case of a migration, I think this should cause an automatic merge rather than reject the migration.

On your proposal, I don't see how the relative dates of the questions is relevant. All of this equally applies to cases where the asker spontaneously posted his question to multiple sites.

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> … is for duplicate questions to be flagged (like duplicate answers are) – yes, that would be an option as well. > I think this should cause an automatic merge rather than reject the migration. – you're right, it would make sense to do that. –  slhck Sep 27 '11 at 7:54
    
> I don't see how the relative dates of the questions is relevant. All of this equally applies to cases where the asker spontaneously posted his question to multiple sites. – Not in most of the cases I've seen. At least for the SO/SU combination, it's always like in the example I gave above. The user doesn't know of SU at all. Of course you could be a bit fuzzy with the relative dates. –  slhck Sep 27 '11 at 7:55

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