Yeah, start with the RSYSLOG. I had that problem as well. What was wrong was that when i added /var/mail.log, it was with other items. So, it would run the mail script for each log file on rotation even though it just provided details for the mail.log.
Make sure mail.log is alone, with it's own set of details in the brackets.
My RSYSLOG for the mail.log addition is:
/usr/local/sbin/postfix_report.sh > /dev/null
invoke-rc.d rsyslog reload > /dev/null
Was mail.log originally in the RSYSLOG file? If not, you might have a duplicate that needs deleted as well. I think that'd be the logrotate.conf but not 100% sure. Can figure it out if necessary.