I have a problem where every Facebook event and birthday syncs as duplicate to Lumia 1020 calendar. Does anybody else have this and know how to get rid of it? I've tried turning synch off and back on and they appear as double again. Thanks M
Go to Calendar → Settings and uncheck the calendar that you don't want to show. This is because some of your FB friends use Hotmail accounts for their FB, so you can unckeck the Birthday Calendar under the Hotmail selection.
For more info click here.
There seem to be a bug in the Facebook app. Only remedy I know of is to uninstall the FB app and reinstall it again.