Well, it WOULD be annoying if Mail / Address Book weren't so smart. When using a big group in Mail, it automatically strips duplicate e-mail addresses. Convenient!
I believe this also works when manually addressing e-mails and across To/Cc/Bcc fields (it won't appear as such, but when you actually send the mail, it won't send duplicates out to those addresses that have been put in the fields more than once). I'm not sure if this is a feature of my mail server or of Mail/Address Book, but it sure does save me time of manually making big groups.
[robg adds: I think it's a Mail feature, as a quick test sending a to and cc to another of my accounts (listing the account twice in each field). I only received one email.]

