Calendar app showing duplicate calendars

Starting this morning I noticed that the Calendar app was showing duplicate calendars. Normally I have three calendars which are Calendar, Birthdays, and US holidays, but now I have two of each. Anyone else seen this? How to delete/remove the duplicate calendars? As a side note Outlook.com>Calendar shows just the normal three. I have the duplicate calendars on both my laptops. So far I have tried deleting and reinstalling my MS account in Calendar. I have remove and reinstalled Mail and Calendar app. I restored an image from last week and initially they were only the three normal ones, but I guess it synced and now I have all six once again. I signed out into a local account and back to MS account. I spent an hour in a chat with Microsoft Support which is always a treat. So far there has not been a solution. I think it must be a MS server side issue, but I cannot do any thing about that. Maybe some MS maintenance has gone rogue. Your suggestions are welcomed.

Check that event and see if it is recursive. Try deleting it then re-adding it to your calendar.

Thanks for you reply. I do not know what "Check that event and see if it is recursive" means. There is not an option to delete the calendar. When I right click one of the duplicate Calendars all I see is a color choice.