An old friend messaged me: his wife’s OS X Calendar app was making duplicates for every entry. She’d create a new entry, fill it out, and then within a moment a duplicate labeled “New Event” appeared. The calendar is synced via iCloud, and it seems like a problem that would be related to that.
Because iCloud sync is live, whenever you create a new event in a synced calendar, the Calendar app pushes the initial raw entry on creation to iCloud, and as you modify it, those updates are sent. It would seem, in this case, that the initial entry is created in one step, and the modified version has a different underlying bit of tracking data that makes it appear like a distinct appointment.
Read more here:: How to stop Calendar from duplicating iCloud synced events