The Duplicates Album may take a very long time to appear. It will only be shown, after Photos has detected some Duplicates. And the search for duplicates is done, when you are not using the app. It needs a lot of processing power to get it done, because Photos is not only checking for identical image files but is comparing what the photos are showing and also showing similar photos. For my moderately sized library with just 60000 photos and videos it took more than a month, before Photos has presented the first few duplicate items.
If you are using a Shared iCloud Photos Library you have to set the selector to "Both Libraries", or the Duplikates Album will not be shown.