I have never had this problem before upgrading to 12.04, but on more than one occasion the front end has launched multiple instances of itself. I first noticed it when watching a video when the audio had a built in echo. I went to close the front end so I could troubleshoot the problem, but when I closed it, I found that another one was still open. With only one running everything worked fine. I didn't think much of it.
I went out of town for business the other day and got a call from my wife about how MythTV had crashed and was unusable. When I got home The frontend was running but taking 5-8 seconds to register a key-press on a wireless keyboard (remote did nothing). So I again backed out of the frontend only to be greeted by another one. I pressed Alt+Tab and the pop up window showed that I had 16 instances of 'mythfrontend.real' open. I closed them all and rebooted and everything seemed fine. My question is how can I prevent more than one instance from running at a time?