Currently Being ModeratedJan 1, 2014 10:29 AM (in response to DJ Luminous)
I had a similar problem with my Calendar and I found that if I tried deleting a list from Reminders on my Macbook it would go away temporarily but when I re-opened Reminders I would see the list again and in some other places the list would multiply such as in the calendar. So, instead of deleting the reminder list from Reminders on my Macbook I deleted the list on my iPhone and for some reason that deleted the list from anything using it and my Calendar was happy again.
Currently Being ModeratedJan 7, 2014 2:26 PM (in response to TGDHersch)
I had the "The request for account “iCloud” failed. The server responded with “412” to operation CalDAVCalendarSplitDualTypeCalendarQueueableOperation." message today after changing and updating my Apple ID email yesterday. Everything worked well until this morning when I received that error message and noticed duplicate calendars in iCal and Reminders. My solution was to login to iCloud, delete duplicate calendars, open iCal on my iMac and all was well again. I hope this might help some, it was a relatively simple fix, for me at least.
Currently Being ModeratedMar 22, 2014 2:48 AM (in response to Lost4)
Logging in to iCloud and deleting it there solved the duplicate calendar issue for me too. I also had the same duplicate calendar in Reminders and deleted that also, moving any events in that account to another account first.