Virtually all AVs recommend only having one resident anti-virus installed.

We preach that and there are still no end of people who have more than one resident AV installed and they get that because they a) don’t know any different, b) don’t check first, so a sticky is hardly likely to reduce this a or b statistic.

avast does I believe check for other AVs during installation but, there are many different AVs which may not be detected until they clash.

So any warning should really be on the download locations to uninstall any resident AV and reboot before installing avast! (but not before you download avast).