Open the Photos app
Tap Albums
Scroll down and tap Duplicates under Utilities
Tap Merge next to each duplicate pair
Tap Select
Tap Select All if needed
Tap Merge to keep one copy and remove the rest
Review the Recently Deleted album
Delete items permanently from Recently Deleted if desired
If Duplicates does not appear, update iPhone to the latest iOS version
If needed, delete duplicates manually by selecting photos and tapping the Trash icon