It seems that if you are seeing this message, something is wrong with your JBoss installation. I ended up getting this error message after a weird sequence of events. First, my computer completely froze up on me -- like a memory problem or something. My application was running in JBoss (through eclipse) at the time. Then, after turning the computer off and back on, I repeatedly got an error when I tried to start JBoss that port 1098 was already in use (I documented the fix for this in another entry: "Port already in use: 1098").

Once I fixed the port problem, I started getting the already installed mbean error. A little research showed that I needed to try either reinstalling JBoss, or messing around with stuff in the deployment directory (the JBoss deployment directory, not the eclipse one). So, I deleted a few folders in there. Then the error went away, but JBoss wouldn't start because it needed those folders. I was able to fix this problem by retrieving the folders from a backup location. Then everything worked fine again.

I wouldn't really recommend this approach. Probably better to resinstall JBoss if you have this problem and can't easily make it go away. The first thing you might check for is if you have your application EAR deployed in both the eclipse deployment location and the JBoss server deployment location.