Need to remove Firefighter ID Owner

I am trying to update the Firefighter (FF) ID Owner and receive the message "Firefither ID is already assigned to firefighter". There is already a second owner in place and I have confirmed that there are no current/active assignments of this ID out to any users. We are still on GRC 10 sp13 but upgrading to SP15 this weekend. How do I go about removing this FF ID Owner who is no longer responsible for this duty?