how do I eliminate duplicate iCal events after synching with iCloud?

I recently upgraded my iPhone to iOS6 and discovered the iCal events on my MacBook were not showing up on iPhone as it had done with iOS5 using iCloud. In trying to remedy, I've unintentionally ended up with duplicate events on all my calendar events. How can I delete all duplicates? How do I prevent in the future? I have OS 10.7.5 and I backup with TimeMachine. Thanks for any guidance.