Billy, are you importing your photos as JPEGs from the iPhone?
If your iPhone is taking your photos in the High efficiency format, but you are importing them as JPEGs, the HEIC files on the iPhone will always appear as new photos, because you did not import the HEIC files. You may want to check the Settings on the iPhone for the Camera and for Photos > Importing to Mac and PC as described here: Using HEIF or HEVC media on Apple devices - Apple Support
Another possible reason for duplicates is iCloud. Is iCloud Photos enabled on your iPhone and "Optimise iPhone Storage"? If the iPhone is syncing with iCloud and continually downloading the photos again from iCloud, when you want to use them on the iPhone, they will be new photos with every download from iCloud. If your iPhone is syncing with iCloud, you should download them from iCloud instead of importing them from the iPhone.