If you see duplicates when transferring photos from your iPhone to your Mac, it's likely because of a mix of transfer methods and format conversion. Try the following tips to resolve the issue:
① Set your iPhone to transfer original photos.
Open Settings > Photos, then scroll down to the "Transfer to Mac or PC" section and choose "Keep Originals."
This way, your iPhone will send photos in their original formats.
② Pick one method to transfer photos from your iPhone to your computer and stick to it.
If you've enabled iCloud Photos on your iPhone and computer, your photos should be automatically synced. However, if you also upload photos via USB, it may cause duplicates.
That said, you should disable iCloud Photos if you want to transfer photos manually. If you want to use iCloud Photos, disable Photos Stream and avoid resending via USB.