Specifically, I'm talking about this question. But if possible, I want a more general guideline on questions that already has answers on other site.
What should we do about these questions?
Right now I'm thinking to migrate to the other site, then let them close as duplicate.
A less preferable option might be simply delete.
Answer them. Don't delete questions just because they have an answer somewhere else.
Some advice:
We don't generally migrate questions to other sites if they are on-topic for Super User.
If there is an answer somewhere else on the Stack Exchange Network it can be used as a source reference for an answer on Super User, just like a source reference elsewhere on the internet.
Remember to follow the rules of attribution (How to reference material written by others).
Tailor the new answer to the question on Super User (don't just copy and paste the source answer).
Don't forget if it is your own question you can Answer your own question
