I did a search and this. When I got to ical folder I found two caches; I experimented and found leaving one in there replicated the problem when I left the other cache file and restarted ical all is well.
This from the fourm
Quit iCal. In Finder, from the Go menu select 'Go to Folder...'. Enter the path~/Library/Calendars/ . A new Finder window should open. From that window move the file Calendar Cache to your Desktop. Re-open iCal.