Leaving the duplicate listing is one option as it really doesn't harm anything.
Another option is to go to another PC and make it a Hub in your Circle (there can be more than one) and try to remove the dead entry (make sure you delete the entry that shows last communication as some date in the past and not a current date/time as removing an active PC will force you to have to reinstall on that PC again)
And finally, you can contact support for help:
How to reach support (FAQ) - http://social.microsoft.com/Forums/en-US/onecareinstallandactivate/thread/30400b52-7f26-4ba0-bc18-17e305329d90
-steve
~ Microsoft MVP Windows Live ~ Windows Live OneCare| Live Mesh|MS Security Essentials Forums Moderator ~