Facebook notifications are shown multiple times
There's a different thread on the official Apple forums that claims to have a solution:


disable all facebook related notification settings in system settings -> notifications.
remove your facebook account from system settings -> internet accounts.
remove all facebook related objects from your keychain and restart your mac.
log on to facebook, you will be asked if you want to use this account for mac system integration, click on later.
in facebook go to settings -> security -> where you're logged in -> delete all old session, especially the ones under "Apple OS Integration".
clear safari cache and restart your mac.
log on to facebook, you will be asked if you want to use this account for mac system integration, this time click ok and check the notifications setting, if everything ok, restart your mac once again.