elmarfromdallas

Same as you delete any photo - select the photo and delete it (on the Mac Command delete is best)

Or maybe as a wild guess the real, un-ask question is how to identify duplicate photos - That is not a feature of Photos although it does detect duplicates during import and when syncing to iCloud Photo Library - if you have created duplicates due to user action then there are three safe, tested programs to identify them so you can delete them using Photos -- PowerPhotos -- Duplicate Annihilator for Photos -- PhotoSweeper for Photos (best and finding similar but not duplicate photos) - other programs might damage your library and should not be used - Using third-party apps to remove duplicate photos might damage your Photos for macOS library - Apple Support

LN