No, they are completely seperate mailboxes.
I just did another test, and it seems its not just CC/BCC mail, its any mail delivered to 2 or more mailboxes on my server.
Eg, if firstname.lastname@example.org
sent an email with headers: To: <email@example.com>, <firstname.lastname@example.org>
Then only email@example.com
receives the mail, not b@myserver
Its weird that in the 1-2 years I've had this server running, I never even noticed this fault!! Bugger!
So any email sent to more than 1 person on my server, only the first person received it, no one else.
Its weird cause in the log, the emails have the same ID, eg: 17E4454EB6