cweil wrote: … look at a duplicates folder in sidebar which does not exist.
The Duplicates view is indeed in the Utilities folder in Photos' sidebar. Your signature indicates you're using the Sonoma OS. The newer Sequoia looks a bit different, so I went searching for a Sonoma screenshot-- if this isn't Sonoma, it's close:

The Duplicates view appears only after Photos finds duplicates. This can take a week or more of scanning. In order to preserve battery life and to not interfere with your work, Photos doesn't do much scanning unless it is plugged in and not being used.
In "Duplicates," Photos shows pictures that are either exact or very similar-- "similar" is why you need to be very careful about deleting or merging them. Here's an example of two pictures that Photos offers to merge
but which are clearly not exact duplicates, and I want to keep them both. The pictures I used at the top are also not exact duplicates--they look the same and have the same titles, but one is higher resolution than the other.
If you want a second opinion in finding duplicates, you can use a 3rd Party app like PhotoSweeper ($15) which works quite well. PowerPhotos ($30) is a versatile Photos accessory app that includes a duplicate finder.
Be very careful about using other 3rd party apps that "Clean" or "remove duplicates." Lots of problems, including corrupted Photos Libraries, have been reported after using some of these.