How many photos do you have? It will depend on the library size.
The "Duplicates" album will be shown in the "Albums > Others" section of the Albums tab, but only, if you are viewing your joint iCloud Photos Library. It is not available, if you are viewing the personal library or the shared library separately.
Are you using iCloud Photos at all?
Try to duplicate a photo to force the Duplicates album to appear.
I do not want to disable iCloud Photos on my iPhone to test, if the duplicates album requires iCloud Photos and a shared library. It would the very wasteful, to disable iCloud and then start the syncing again. It would be great, if someone could test, if the Duplicate album will appear, after turning iCloud Photos and the shared library on.
Find and delete duplicate photos and videos on iPhone - Apple Support (BH)