Yes. The guy may not want to be destructive but then may try to use the popularity of a domain.So what happens if there are common usernames for a given domain on both the servers. Where does the mail xyz.domain.com go where the domain.com is set on both licensed copies on different servers. Oh its getting so confusing for me. But yes thats what I was precisely wanting to know.
In short, I think GM should have a builtin restriction that the domains from the same server (where GM is installed) can be included. This would eliminate such nasty attempts if at all.