I have not so much found the solution to this but more found the reason behind it.
Not a very technically worded answer but its still something and I may not explain it very well, sorry!
Some mail servers run checks on the domain the email is being sent to. So if you have personA@company1.com and personB@company1.com and personZ@company2.com. It locates the 2 different domains and sends the mail off to the appropriate server for company1.com & company2.com. The receiving server then handles relaying it to the specified people.
However, I have also found that some mail servers dont run this check so to speak and cycle through the email addresses and find personA@company1.com and sends the mail to the server, moves onto personB@company1.com sends the mail to this server and then personZ@company2.com and so on.
The receiving server then takes this mail, for PersonA, reads it and sees its going to: personA and moves that on. Also sees is going to PersonB and moves that on and ignores PersonZ. Then it gets the other mail through that was for PersonB. It moves this mail on to that person, reads PersonA and moves that on and ignores PersonZ. So as a result Person A & B both received the same email....twice!
Something to about this is on the microsoft website somewhere but its a damn nuiscance to find...i will try to locate the URL again.
- FateFirst