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(When) is cross-posting ok?

This community sometimes gets posts that are copied from other sites.  We delete those because copying from somewhere else is plagiarism.  How do we want to handle cases where the *author* does the copying?  If somebody says "I am the person who wrote that Quora/Reddit/SE/etc question and I want to ask it here too", what do we want our policy to be?

To avoid the appearance of plagiarism I think the post here would need to, at minimum, say something like "I previously asked this at (link)" and include the link.

Is that sufficient, or do we have other expectations?