*** Answer improved by Wrobinson on 12/17/07 ***
Please refer to the following MSKB article http://support.microsoft.com/kb/820735.
*** End update ***
Im not sure if it is. If the user is a member of the group and they are cc too then by common sense they would recieve twice. The user sending the original message could check if the cc;d user is a member of the group before cc’ing i suppose. I have hundreds of groups which are lsited in Excel for users to see if they are included. The excel sheet is up on sharepoint and I have only accassionally come across this issue. However once the user complains about getting 2 emails the sender normally remembers not to cc them again.