I don’t have a duplicates function under photo/utilities (iphone 16.5.1)?

Starting up from the bottom, *IF* the phone is connected to iCloud Photos, then you *DON'T* delete photos from the phone without deleting them from iCloud Photos, because the business of iCloud Photos is to keep itself in sync with the devices connected to it (and them to each other) -- adding, changing, or deleting as needed to make that happen.

Second, that doesn't have much to do with the iOS 16 Duplicates feature, which deals with duplicates and near-duplicates on the device. The Duplicates feature may take a while to come up with a list of candidate images, since it essentially has to compare every image in your library with ever other image to some degree.

Until it has built up some candidate images, the feature will be invisible. When it does appear, it will be in Albums > Utilities > Duplicates.