iCal slowly duplicates entires after importing an ics file

After downloading an .ics calendar file for my local sports team schedule, iCal slowly doubles each entry one by one.

Is there some setting somewhere that is doubling or duplicating the data?

It imports correctly, then eventually duplicates each as it uploads the new events into iCloud.

This is on the latest iCal, Mac OS X Mountain Lion, iOS 6 setup.