It only appears if duplicates are found.
Let the photos app settle some time after updating and it will look for duplicates and - if it finds some - you'll start seeing the duplicates folder again.
This option is in macOS Ventura, Sonoma under the photos app and ios 16 for sure but I don't know if it is available in the latest version.. if it is not available in the latest then need to use third-party apps like: https://www.duplicatephotosfixer.com/ or any other you know