merge photos

Your "Duplicates" Album does look like a Standardl album, not like the "Duplicates" album in the Library section. It is strange that you are just getting a flat list of 367 Photos and the options "Photos Play Memory Video Slideshow" instead of neat groups of Duplicate pairs with the option to merge. Did you really click the "Duplicates" in the Library section of the sidebar (below "imports") and not a different album in the "My Albums" section?

Great!

It really takes a lot of time to find the duplicates. Photos is not just comparing the file attributes and looking for identical, duplicate files, but scanning for similar images and lookig at the contents of a photo. The user guide is warning, that we will have to wait: Remove duplicate photos in Photos on Mac – Apple Support (UK)

But it is an understatement: "Depending on the size of your library, duplicates may take some time to appear as Photos analyses your photos.)" I had to wait much more than a week for the duplicates to appear.

The option to merge duplicates will be shown in the Duplicates album created by Photos, once Photos has scanned the library for duplicates. You can only merge duplicates suggested by Photos, not arbitrary duplicates selected by you. It may take several days, even weeks, before Photos will show you duplicates to merge.

Once Photos has detected duplicates, you will see the "Merge" button above the suggested duplicates, to the right of the capture date of the duplicates, for example:

Because when Photos scanned the library it deemed them to be duplicates.

As those images are not identical, there will be no ability to merge.