Today I launched iCal and noticed that all my entries were duplicated. Removing unnecessary copies would be so time consuming it’s not even funny. After I opened Calendars List, it showed two sets of calendars – On My Mac and iCloud. Unchecking every calendar in one of them, even thought seemed to help, was not a real solution. Luckily real fix is equally simple. It worked for me, hopefully will help someone:
Quit iCal, go to System Preferences and navigate to iCloud. Uncheck Calendars, confirm and then check Calendars again. Confirm again if needed and launch iCal. You should only see a list of iCloud calendars and no duplicates!