Cross-posting is strongly discouraged on some Stack Exchange sites, such as SoftwareEngineering.SE. There:
If the question appears to belong to another site (which happens quite often with questions which should be on Stack Overflow), users vote for the question to be migrated if its quality is good enough (or closed otherwise).
If the question is already posted by the same member on Stack Overflow, the author receives a comment explaining that he shouldn't cross-post. This, usually, follows by a lot of downvotes and close votes (although it may be that people who cross-post there are also often the ones who write bad questions in the first place).
I see that there is an opposite practice here, especially in relation to the questions about Arduino. Example:
Ask on SE.Ardruno, not here.
Is this an official practice here?
If not, would it be possible to stop inviting the users to cross-post, as the same users may then cross-post on other sites where the practice is discouraged?