This might help, it solved my problem:
In this situation, the person was asking how to remove duplicates from their iPad. I was having the same problem on my iPhone as well as my MacBook. The instructions worked anyway.
Go to settings on your device, iCloud, Calenders: slide to off. Delete all events when asked. Wait 1-2 seconds, then slide Calenders back on. Merge all events when asked. Then check the Calender app. If it does not work, go to Calender on you Mac, and refresh the Calender. Then repeat.
I had to do this procedure twice before the duplicates went away, but at least it worked. I don't understand what is going on; it's like the system gets clogged up and you have to clean it out. Hope that helps.
If there is a better method, someone let us know.