My iPhones and iPad are synced with my iMac via iCloud. All devices work perfectly except iCal on my iMac shows all entries in duplicate.
Adding a new entry shows up as a duplicate in the iMac but as a single entry on all other devices (which is correct)
Here are two troubleshooting articles which may be helpful for you:
1. iCloud: Advanced Calendar and iCal troubleshooting
2. iCloud: Resolving duplicate calendars after setting up your calendar application for iCloud Calendar
Retrieving data ...