I had a similar issue. After upgrading from Lion to Mountain Lion, nearly all entries in iCal were duplicated...but ONLY on my iPad, and only the entries I put on the calendar (my wife and I have some shared calendars: home/Work/Personal, etc. Entries or events she put on did not duplicate, nor did birthdays.
First, to check it out-
I logged in on icloud.com and duplicate events WERE NOT present there. They were only on my iPad and not on my iPhone or my wifes iPhone, and not on either of our iMacs either.
I did not have the "Sync" buttons checked in iTunes either.
To resolve the problem I went into: Settings>iCloud (on my iPad).
Then I Turned OFF the button for calendars, and told it to delete the info from my iPad. All calendar info then disappeared from my iPad (except birthdays). I then clicked the calendar button in: Settings>iCloud to turn it back on. All events and entries reappeared and WERE NOT duplicated!
I have same problem, but unrelated to iCloud. I have iCloud turned OFF so how do I get ride of so many duplicates? I even tried saving calendar entries, deleting all cal's, syncing iphone to iTunes with Cal checked to sync-it deleted all off iphone...so I then put my single set of 3 cal's back into calendar on desktop (the backups, imported) and re-sync'd...now again, have MULTIPLE entries on iphone and desktop cal! So it is getting these from someplace on my desktop. Where are they all stored, and what exact file names to delete to keep newest set and get rid of all duplicates?