Duplicate Face Names

To combine the two just drag the name you don't want to use onto the one you do and it will merge the two.

Photos is using the "first name only" for all people, where you assign the name using the suggested names from your Contacts and there is not yet another person with this first name. You cannot prevent this, if you want to link the named people to the Contacts, so Siri can find your photos. When I want to be sure, that the names will not be reduced to first names I am breaking the connection to the Contacts by replacing the space characters in the names by underscores. This way Photos cannot recognise the first names and I can see the full name in the People album. I have to do this for the Photos Library I am using at work. It is not very professional to address each and every professional contact by the first name only.

The duplicate entries in the People album are created, when you name people by typimg a name into the name field below the faces circle and do not pick the same suggestion from the list of suggested names as before. If there are several names for the person in the list of suggested names, it is easy to pick a different name, and that will create the duplicate. When I create a new person in the album, I will immediately pick a distinctive key photo, so I can easily recognise the correct name in the list.