There have been many questions that ask if a user is allowed to cross-post a question on multiple sites, and the general conclusion is that they are not allowed to do so. However, sometimes there are questions which are asked on multiple sites by different users not knowing another SE site had an answer already, where it is on topic on both sites, that have some overlap. What is the policy for such questions? Should they be closed on one site, migrated, or is each site independent enough to allow the question to remain on both sites?
Each SE site is its own island, to an extent. Sure, the rules are pretty much the same, but you will get a different group of answers from one group compared to another. Furthermore, it seems unlikely that a new user will have the complete knowledge about every Stack Exchange site. It would be too much to ask for them to have such knowledge.
Furthermore, you can always link to other questions and answers, in the same way that one would link to Wikipedia.