It is pretty sad when such a minor issue (which should never happen in the first place, at this stage of the game in a product's lifespan) is allowed to be let go for so long before its fixed. I personally, know Avast is better than this... but it doesn't say much for them to new people who haven't used their product very long. Newbies can easily think "If you can't fix something as simple as a double notification, then how can I trust you can do something as complex as protecting my computer?"
You're better than this Avast.... FIX IT!
P.S. All my machines (Win7/64 & Win XP Pro) are STILL doing it