Have you already tried to reinstall all apps, that are creating duplicates on import? At least a previous version of WhatsApp had a problem, that prevented photos from deleting them and caused duplicates.
Another known reason for photos being duplicated is a format conversion or syncing with iCloud. The duplicate detection in Photos on your Mac is comparing the file attributes. Only exactly identical files are considered as duplicates - the filename, the creation date and modification date need to be the same. If you are downloading converted or synced files from the iPhone, they will appear as new files. the duplicate detection will be disabled in the following situations:
- If the Setting > Photos > Transfer to Mac or PC is set to "Most Compatible", because files will be converted to JPEGs before downloading to the Mac, while the originals in a different format remain on the iPhone. Disable any format conversion, for example: (Using HEIF or HEVC media on Apple devices - Apple Support)
- If any cloud syncing is enabled (Settings > Your Name > iCloud > Photos) - do not use iCloud Photos or My Photo Stream, if you do not want to have to check for duplicates manually.
Any app, that is keeping the photos online and you have to download the photos again, when you view them, will create duplicates, if you have to download the same photo several times. overtime you view the photo in the App and it is saving a copy in Photos, you will create a duplicate, when you download the photos manually to your Mac.
