How to purge calendar entries

I wouldn’t purge everything yet. Duplicates that can’t be edited often come from another account or a subscribed, shared, or read-only calendar.

On the iPhone, open Calendar and tap Calendars. On the Mac, choose View > Show Calendar List. Turn off one calendar at a time. When the duplicates disappear, you’ll know which calendar is causing them.

Before deleting anything, export any calendar you need to keep from the Mac using File > Export > Export. Deleting an iCloud calendar removes its events from every device using that iCloud account.

Which accounts are listed there—iCloud, Google, Outlook/Exchange, or “On My Mac”? The cleanup steps depend on where the duplicate events are stored.

That helps. I wouldn’t delete the items under Other. Birthdays comes from Contacts, Scheduled Reminders comes from Reminders, and Siri Suggestions and Found in Mail are generated automatically. You can uncheck those to hide them. The Holiday calendars are also read-only, so uncheck them one at a time to find the duplicate.

For the regular calendars, I’d do the cleanup on the Mac. Create one new iCloud calendar first, then Control-click each old calendar and choose Get Info before deleting it.

Be especially careful with the two Family calendars. One may be the shared calendar created by Family Sharing, or it may belong to someone else. Deleting a shared calendar could affect other family members.

Once you’re sure a calendar is yours and no longer needed, select it and choose Edit > Delete. Then give iCloud some time to sync before checking the iPhone. iCloud also keeps recoverable calendar archives under Data Recovery if something important is removed accidentally.