I am having the same problem. I use iCal as my main calendar on my Mac but sync to my iPhone and iPad. Some of the calendars are iCloud calendars and behaving just fine.
However, the Google ones are not and show duplicate entries.
On the iPad and iPhone, there are no duplicates. If I log into Google and look at the calendar there, there are no duplicates. Occasionally, things correct themselves on the iCal on the Mac and no duplicates but then, randomly, they come back again.
A solution would be very welcome. It's driving me crazy.