Photos app on my iMac running Ventura 13.4 does not scan for duplicates...

The idea is that Photos doesn't want to do the scanning while you're working, because that would slow down its response to you. The scanning is a background process, so it continues even when Photos is not open. Nonetheless, even though it may not matter, when we think that something has changed that requires scanning, most of us leave the computer running overnight with Photos open in the background. You may also have to be wearing your lucky socks.

I've never seen a Duplicates folder with no duplicates in it. It sounds like something's wrong. First thing I would do is create a new Library. With Photos closed, option-click on the Photos app and choose "Create New." Then put a few pictures in it including duplicates (like the same picture with a different file name) and see if this new Library works detects the duplicates. That will let us narrow down the problem.

Let us know what happens...

How big is your library? It can take several weeks for Photos to complete indexing the library for objects, people and duplicates. In a test on a library that had finished syncing and removing all duplicates, I added a couple of duplicate photos to the library. It took over a day for the Duplicate albums to appear with those new duplicates. It's not a fast process.

If you want to use a 3rd party app for identifying and culling out duplicates try the following:

You want an app that will identify the potential duplicates, put them in an album or mark them with a keyword for easy retrieval and deletion by you. You don't want one that does the deletion itself for obvious reasons. 

I've run tests on the these two apps with the following results and found them to be safe to use:

PowerPhotos - $29.95  

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos, both original and edited versions, along with their metadata between libraries.

PhotoSweeper - $9.99 - Demo version available.

PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different file sizes, file names, image sizes and capture dates.

Personally I prefer PowerPhotos as it's a very powerful utility for Photos libraries.

For Photos to do its scanning it should be open but in the background, i.e. with another app like the Finder first and foremost.