I've seen this a lot. Although I do not know the cause, I know how I have dealt with it in the past. When you type in a name, wait for the list of possibilities to display and then select the name from the list. Any time I've just typed in a name and pressed Enter, it creates a new list entry, and if you go back to the main People page, you will see more than one instance of the person's face. It also helps to assign a "Key Image" to each person. Double-click on the person, select the "show more" link, right-click on the desired image and select "Key Image". Then, when you are adding a name to a photo, it will (should) show small icons of your selected key image faces beside each displayed name in the list. Also, it the event that the person is in your contacts, use the key image to assign a photo to their contact information. Then you can be sure you are selecting the correct name when multiple duplicate names appear in the list when adding in a new face name. You can clean up the duplicates: Go back to the main People page, locate the original and any duplicates. You can combine these by dragging the duplicate (last first), on top of the original, and answering Yes to combine them. After all the duplicates have been combined with the original, there should then be only one name show up when typing in a new face label.