I'm from Worldbuilding.SE, and we could use the ability to close as a duplicate of another site.
It may be unusual for some sites. It may even be unusual for most sites. But it's common to have questions asked at Worldbuilding.SE that have been adequately (even marvelously) answered on another site. There's so much overlap between us and other sites (where the most significant difference is context) that we've had to adopt a real-world question policy.
It would be very beneficial to have the ability to close a question as a duplicate found on another site. The very same mechanism can be used, it simply would allow us to look at more than questions from our own site.
It minimizes the duplication over many sites.
It cross-promotes sites. People may not realize (and regularly don't) that if they strip the "I'm asking about my fictional world..." part out of their question, it can be asked on another site — and possibly even get a better answer. People would see the many sites that touch on the worldbuilding process.1
Note that the original question to which this is an answering post considers a wider range of issues than I have in my answer. Sometimes a question should be migrated because "it's off-topic here." I'm specifically addressing the issue of, "it's on-topic here, but this question has already been asked over there."
Recommendation: enhance the close-as-duplicate process to permit questions from all stacks, not just the current stack in question. If necessary, set a unique rep limit for this ability. I.E., at rep level #1 you can VTC as a duplicate from questions on your home stack. At rep level #2 you can VTC as a duplicate from any stack.
1 The astute observer may then ask, "why do we even have a worldbuilding.SE?" I answer, "because there isn't a place where people can ask about how to develop a consistent magic system, or an alternative physics system, or even a place where people can ask about how to create an anatomically correct Orc, elsewhere in the SE universe."