Birthdays calendar behavior

lucafabbri365 

Yes, unfortunately, that is the current designed and expected behavior. This is because they are 2 separate features controlled at 2 separate levels.

The Birthday Calendar feature is an Exchange level feature exposed in Outlook on the Web (OWA). It creates an additional Calendar which is synched to Outlook Desktop as well. The entries are based on the Birthday information of your contacts in the Contacts folder.

The Birthday events that are being added to your main Calendar folder is an Outlook Desktop feature. These events are created when you add or update the birthday field of a contact and then save the contact. The same applies to the anniversary field.

Unfortunately, unlike in Outlook on the Web, there is no option to disable this behavior in Outlook Desktop and thus could end up with duplicate birthday events and have to delete them manually from your main Calendar.