This question is being written from my son's email address, for reasons that will become apparent below.

My son has had a personal Live account set up for him for the last three years.  For the second time we have suffered an instance of other users emails being delivered to my son's in box.  I immediately changed the password for access again and started sorting through the errant email addresses, mostly sign ups for on line game sites. 

However, a couple of emails appear to show that my son's email address has also been assigned to another user, a total stranger living some 300 miles away, the email in question showed the other persons home address and full details.  It appears that both this stranger and my son have exactly the same email address assigned to them which I find strange as when his account was set up it checked that there were no duplicates and that the address was free.  I'm at a loss how this has happened or what course of action MS can take to correct the issue because as far as I am concerned one of the users would have to close their account, and that's something I'm not willing to address at this point in time.

Any suggestions gratefully welcome.