How do I remove duplicate photos and videos from my iPhone

Give photos plenty of time to scan your Photos Library for duplicates. Keep the iphone connected to a power source and the internet while you are not using it.

Once Photos has scanned all your photos you should be seeing a Duplicates album.

Then you can merge the detected duplicates automatically as described here: Find and delete duplicate photos and videos on iPhone – Apple Support (UK)

This feature is only available in iOS 16.1 or later, and it may take several days or even weeks before the Duplicates album will appear.