I'd think, using Win 7, if you set several different themes to each run the avast! screen saver, then maybe you might wind up with more than one entry?
Win 7 does have quite a few downloadable screensavers on the microsoft site, went and got a few of them but only set one to run the a/v screensaver scan. I wound up with only one. This also happened on XP a couple of times after setting the a/v scan to run that way, so...
Maybe that is the case here?