why 2 birthday events in calendar?

If you look at the example below, you'll note there are two entries for Leeann's birthday. The one with the little gift symbol beside it was entered as part of my data for that particular contact. The other entry was entered via the calendar app. If you see the one with the little gift symbol, double click it to reveal more info, including at the bottom "Show in Contacts".

If you have added birthdays, or other events within the Contacts App they will appear in your calendar. This could explain the duplication. If you check the details for each of the duplicated event, it should show the source of the info.

That means you have multiple entries in Contacts or entries that have birthdays in them that do not belong to the actual contact person.

Perhaps you have entered multiple birthdays in their Contacts card. Have you just tried deleting one of the birthdays from the Calendar?

Also check in your System Preferences, Internet Accounts, to verify whether duplicate dates may be coming from a Facebook account.