When you enter a birthday for a contact, Organise You can add a yearly Birthday item to the calendar.
Before creating another birthday, Organise You checks for an existing likely match. If it finds one, it can link the existing birthday instead of creating a duplicate.
If you later change the contact's birthday, the linked calendar birthday can be updated too. If one year has already been moved with This occurrence only, that moved birthday remains the replacement for that year rather than allowing the corrected recurring date to appear as a second birthday.