Finding something really odd with Avast virus signature updates: I get a vocal announcement that the virus database has been updated but no pop-up message box; then, about 15 secs latter, I get another vocal announcement, this time accompanied by the pop-up message box!
it is a known bug…many posts in here if you search
like this one http://forum.avast.com/index.php?topic=102073.0
There is currently a beta version being tested and amongst other things it resolves this problem.