From John C:
There are several Contact entries whose birthday is duplicated in Calendar. There are no duplicate entries in Contact.
The only calendar enabled in Calendar is JC Home. In Mail, the iCloud account is the only one that enables Calendar. The other accounts are enabled for Mail only.
Troubleshooting steps:
1. Toggle the Birthdays Calendar Off and On: This forces the Calendar app to clear its cache and rebuild the birthday list from scratch.
2. Open Calendar and go to Settings (or Preferences) from the top menu bar. Under the General tab, toggle Show Birthdays calendar.
Either one of both of the above troubleshooting tips will resolve the duplicate birthday entries in Calendar.
Another condition for duplicate birthday entries is when two or more email accounts are enabled for Contacts. This would show duplicate entries in Contacts, so be careful when deleting entries in Contacts. It is best to have only one email account enabled for Contacts to avoid duplicate birthdays in Calendar. If more one email account has Contacts enabled, then in Contacts there will be an additional field that identifies which account that contact is for.