I had the same problem. I updated to IOS5 on my iPhone 3G last night, then decided this morning to update to Lion on my MacBook (kept meaning to but just never did). Calender events were all okay on the phone, but I had duplicates on iCal on the macBook. This is how I resolved it: clicked on iCal, Preferences and then Accounts. I saw an account for both icloud and mobile me. I deleted the mobile me account and it deleted all of my duplicates. It also deleted the Us Holidays calendar I subscribed to on my iPhone, but I subscribed to that again, this time under iCloud and all is good. Hope this works for you!