Are you saying that Photos isn't finding your duplicates and putting them in the Duplicates folder? Duplicates are hard. Sometimes it takes a long time for Photos to find all the duplicates, and sometimes, in trying to not mistake too many "similars" for exact copies, it will miss some.
You could see if another app helps. Be careful, though--there are many 3rd party apps that can really mess up a Photos Library. I (and lots of us) use PowerPhotos ($30) to "accessorize" Photos-- this is a trusted app that has been used with Photos for, really, decades. It does duplicates, and it may find copies that Photos misses, and it may miss some that Photos gets. I use it for duplicates when I'm impatient for Photos to find them.
Let us know if you try it and it helps (or not.)