If a question is asked on one of SE site and ditto same question is asked in another SE site,
- How can one find out?
- How to find out if such duplicates exist?
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Just hit the search box in top right and it will search across all sites. Try to search as specifically as your question is, then broaden it if you don't find any duplicates.
If / when you do find a cross-site duplicate, this post can then give advice on how to handle it.