I am really struggling with duplicate entries within iCloud.
Follow the Resolution steps in: iCloud: Resolving duplicate calendars after setting up your calendar application for iCloud Calendar.
Are you seeing them on your Mac (in iCal) or on the website at www.icloud.com
I followed those steps and my problem was referred to iCloud support.
On both, I downloaded calendar dedup and it deleted all duplicates on iCal, however the duplicates that are in iCal looks like they are referencing iCloud and I can't delete them from within iCal or iCloud.
iCal will have 2 accounts, iCloud and On My Mac, view iCloud only and see if there are dupes,
Retrieving data ...