YES when avast is not installed, avast previous versions DO NOT SHOW UP. If Avast is installed in the computer and you run CCleaner the two items of Avast shows up.
About superseded with a new one, I reformatted my hard disk just to make sure. I then uninstalled Norton (pre-installed), ran it's uninstaller utility, and installed Avast and rebooted my laptop. Installed and ran CCleaner and BINGO the two items of Avast 6 and 8 shows up in CCleaner even though that was the first time I installed Avast.
Also at that time I also installed Macromedia Shockwave Player and BINGO the two version of shockwave player shows up in CCleaner.
Looks to me it's a bug in CCleaner. Prove is in my first message of this post I took it directly from Avast Forum