Single-photo People

For example:

The Dobby faces album has been looking like this for many months:

After duplicating the photo and retouching the background a bit in the copy, the album is now showing two photos and is working properly:

"Optimise Storage" is one of the options for iCloud Photos. It does not apply, if iCloud Photos is disabled on your Mac.

One My Mac some people albums with only a single photo settled down after a few months the message about "updating" vanished automatically for them. On others I had to use brute force and to add a second photo.

What I did was to edit the photo slightly, export a new version of the photo with "File > Export > Export ... photo" and pick a very high quality and original size, then reimport them. If the photos have been very small (scans sent to me by Mail and with a small width and height), I enlarge the pixel size with the online Image Enlarger (https://www.imageenlarger.com) and imported the larger version as well and tagged the face there. Soon after I added a second face using a larger photo, the message vanished and I could open the album for this person.

Or duplicate the existing photo (if it has a reasonable size). Simply use "Image > Duplicate", the open the duplicate in Edit Mode and use the "Retouche" brush to brush away a minor detail that does not matter. Now close the edit panel, then name the face in the duplicate as well. Wait a bit, then open the People album for this person by clicking the thumbnail below the enlarged photo.

Which system version do you have installed? Are you using iCloud Photos and Optimise Storage?