(For the pages with similar URL structure: ) It's almost like Google thinks that your site and the other site(s) are really the same bunch of files on a single server being returned under multiple domain names. They then test this out by requesting those pages from each domain and see what they get back.
You should return 404 for those requests to show that the other sites are separate sites. If you were to somehow "adopt" the URLs I think some sort of mayhem would ensue. I wonder if pages from your site are being requested on those other sites?