Family Sharing: Account Already in Family

I am trying to set up Family Sharing. When I enter my account information to be the organizer, I receive an error message stating: Account Already in Family: This account is already shared by another family member. Family sharing works best if every family member uses their own account." This is the first time I or anyone in my family has attempted to set up Family Sharing. Any idea why I would be getting this message or what I can do about it?