Does this tell me that other people are using my proxy service or that my clients are having multiple connections on port 80?
Yes you probably have not secured it form users out on the Internet using your proxy. Get the firewall up ASAP! MikroTik have docs on securing your router
on the webproxy access allow only your internal network and deny all..
ip web-proxy access > add src-address=10.x.x.x/24 action=accept
then add action=deny
Regards,