Hi there CateBee!
I have some information here that may be able to help you with that issue:
Birthdays that you manually entered in Calendar (or iCal) can sometimes result in issues, such as infinitely repeating events that can cause slowness and contribute to unnecessary resource utilization in Calendar (or iCal).
While you can add birthdays to any calendar in Calendar (or iCal), it's better to use the Birthdays Calendar. The Birthdays Calendar is a calendar that is automatically created from the birthday information found in Contacts (or Address Book). It's easy to update the Birthdays Calendar by updating your Contacts (or Address Book) with the birthday for each contact. The Birthdays Calendar is enabled by default on iOS. For more information about the Birthdays Calendar, see iCloud Calendar Help or OS X Address Book help.
To clean up issues related to manually adding birthdays to a calendar in Calendar (or iCal):
- If you already have the Birthdays Calendar enabled, deselect it in the Calendars list in Calendar (or iCal).
- Search Calendar (or iCal) by entering "birthday" (without the quotes) in the Search dialog in the top-right corner of the Calendar (or iCal) window.
- Sort the Search results by title, so they are easier to analyze.
- Locate any birthdays that are returned in the Search results that are not contained in a Holidays Calendar (such as John F. Kennedy's Birthday, Lincoln's Birthday, and so on).
- For each non-Holidays birthday returned, decide if you want to keep it. If so, open Contacts (or Address Book) and verify that you have a Contact for the individual. Enter the birthday information for that person in Contacts (or Address Book). For more information about updating Contacts (or Address Book) fields, see this article.
- After verifying that the birthday is entered in Contacts (or Address Book), remove any birthday events not stored in the Birthdays or Holidays calendars by selecting them and choosing Delete from the Edit menu.
- Note: You can select consecutive items by holding down the Shift key while selecting them. You can select non-consecutive items by holding down the Command key while selecting them.
After cleaning up any manually added birthdays, you can enable or re-enable the Birthdays Calendar. To enable the Birthdays Calendar for the first time, open Calendar (or iCal) > Preferences and select the checkbox next to Show Birthday's Calendar. If you temporarily hid the Birthdays Calendar as per the instructions above, you can re-enable it by selecting the checkbox for it in the Calendar List.
This information came from the following article:
Thanks for coming to the Apple Support Communities!
I had the same issue with birthdays showing up twice and it's sometimes hard to remember where I got them from as some are from syncing with Facebook, LinkedIn etc.
What I did was to disable Show Birthdays in iCal preferences rather than simply unchecking show Birthdays subscriptions in iCal sidebar.
I restarted iCal and checked Show Birthday calendar in ICal preferences again. Now all my doubles are gone.