I use Mailwasher differently (as do a number of others I know). Mailwasher - Mail Client > avast > server/inbox
4.1 - I run MW first (no change to original account settings, no 127.0.0.1 changes, etc.), this deletes email from the server and hands off to OE. I then download remaining emails left on the server and avast scans them into OE inbox.
So there is no checking of the small portion of each email MW downloads to analyse and no possibility of double avast headers. This however, may change with 4.5 (I can't say I have yet to upgrade to 4.5 beta).