avast supports multiple ports to be redirected, there needs to be a comma separating the ports and you must uncheck the 'Ignore local communication.'
These settings are for the web shield if you do the manual settings in your browser to use the webwasher proxy, then that browser won't be using port 80 so there is no duplicate scanning, because you are using a local proxy, that is a local communication which needs to be scanned.
Leaving the port 80 in the web shield shouldn't hurt but you can remove it, I would tend to leave it there as a belt and braces if you use another browser and you haven't set that to use the webwasher proxy or something else tries to use port 80 it will be intercepted.
If you change the browser to use port 8080 yet leave the web shield set to redirect only port 80 and leave 'Ignore local communication' set, then avast won;t scan the webwasher content.