If you are seeing Birthday events on your calendar that are duplicates or that you don’t want to see, it’s likely a setting with your calendar provider is causing it.
For Google Calendar:
Sign in to Google Calendar in your desktop browser
Go to My Calendars > Settings > Birthdays
Customise the birthdays you wish to see
Sign into iCloud.com in your desktop browser
Click on Calendar then tap the Settings cog icon on the bottom left
Choose to tick or untick “Show Birthdays Calendar"
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