From the FAQs
I have more than one Official Membership in my name - what do I have to do during the Fan Update process?
Our terms and conditions of membership state that members should only have one membership account in their name. Therefore, it will not be possible to have more than one membership account, whether this is an Official Membership, Season Ticket or other account, with the same details. Should you have more than one Official Membership, Season Ticket or other account in your name, then we will offer you this one-off chance to update those accounts with another supporter’s details.
Be interesting how this works with all the STH not in the person's name etc