Normally when I go to the Market there's a list of apps and if I already have an app, it'll say "Installed." Also when I go to Downloads I get a list at the top of what has updates. That's pretty normal for most, if not all, of us.
Yesterday I went to the Market and it had an app called Voice Search. That came with the Droid, I believe, and have had it for some time. It didn't show installed, it showed available to install. At first I thought it was another app (although it had the same icon as what I had). I downloaded it.
It was an update to the app I already had, but it didn't act like what I described in the first paragraph. The UI looks different, but it surprised me when I press the icon that I've had for some time and the new one showed up.
No biggie, just wanted to see if that happened to others.