As before, Duplicates is in the Utilities folder. If no one's changed it, then Utilities is at the bottom of the Main Window-- you may have to open it up to see what's inside. (The location of Utilities can be changed or hidden with the Customize & Reorder menu at the very bottom.
The Duplicates view shows pictures that Photos has found to be similar or identical to each other. If it hasn't found any duplicates, then the Duplicates view won't be there. To find duplicates, Photos has to compare, in some way, every picture to every other one, and this can take quite a while. How long has it been since you updated to iOS 18? It could take a week, certainly. But to avoid battery drain, and to keep from interfering with your use of the phone, the scanning best happens when the phone is plugged in and left alone. Several overnights is good. It's possible it works better plugged in through the USB-c port.
You could try duplicating a recent picture and see when it shows up…