This topic was already discussed in Allow cross-site duplicate closing feature request. It was badly received and I generally agree, but it has some valid points.
So far conclusions that was not disproved are, in favor of off-topic:
- If question is off-topic on the site, it should be closed as such.
- If questiin is on-topic on more than one site, each site will provide answers to different aspects of a problem.
On the other side, in favor of duplicates:
Closing as duplicate would help redirect future users to where question belongs and is answered.
Deleting off-topic questions means they are not searchable, and users post again.
Closing with off-topic message
Questions on programming, PHP, SQL, etc. that do not relate directly to Drupal are off-topic here, but can be asked on Stack Overflow.
results with question posted again on Stack Overflow and closed there, this time as a duplicate, increasing user's frustration.
The way to solve this and satisfy both sides would be:
For close reasons saying to ask on another page (and custom ones), provide optional field for cross-site duplicate link. Of course Stack sites only. Change message text to something like:
Questions on programming, PHP, SQL, etc. that do not relate directly to Drupal are off-topic here, and this one was already asked on Stack Overflow [link].
It should work as regular off-topic with following changes:
- should be exempted from automatic deletion,
- should provide the same redirection (if any) as duplicates.
That way questions about HTML, CSS, PHP and so on will not appear and disappear on CMS related sites, as people interested in programming or styling something in that particular CMS will see a result in their search and will be directly send to the proper page. What more, person who posted off-topic like that would be sent directly to answers, so he will have to deal wit only one closure and will see his answers sooner.
This would also be an easier and cleaner way to handle the situation where new users, upon learning a different site is better for their question, cross-posts before the question can be migrated.
Important note: I'm not talking about user or user group reposting off-topic questions once they have been told it's off-topic. I want to help honest users that are asking in good faith, posting on Drupal Answers or WordPress Answers what should be posted on Stack Overflow because they have put their problem in search and didn't found any results. Usually question about the same thing was asked by another user, but it's now long gone deleted, thus invisible in search. It can be helped.