Same issue here
I have a mail address setup to forward to 4 other mail addresses (as account-less mail list are not possible yet).
When receiving a mail to that list, even though the antivirus is setup on it, it forwards the mail to the 4 other accounts, and because of this the mail is scanned 4 times - and passes through spamassassin 4 times as well.
Could it be possible to have the forwards done AFTER the scans ?
And then use added headers to not run those twice (that's just an idea - not sure what it's worth regarding security, IE if the sender set that particular header we should remove it first) ?
The issue here is that this takes a lot of CPU (4 scans) , and when a virus is found a mail is sent 4 times (once per account) to the mail list saying a virus was found (disabled that for now).