Photos questions here! Duplicate photos issue, huge! issue.

There are two common reasons for already imported photos not being recognised as duplicates. Some Settings on the iPhone can be causing this:

1. Reason 1: iCloud Syncing: If you have iCloud Photos enabled on the iPhone, the iCloud syncing may be turning the local copies over and over again into "new" photos. Please check, if iCloud Photos has become enabled.This can happen after a system update as a default, without you being aware of it. Is "Settings > your Name > iCloud > Photos > iCloud" enabled? Or is My Photo Stream enabled? If yes, do not use the import via a USB connection without checking the new photos for duplicates. If photos have been downloaded again from iCloud, Photos may not be able to recognise them as already imported.
2. Reason 2: A format conversion: If your iPhone is taking the photos as HEIC files, but you are importing them converted to JPEGs, the original HEIC files on the iPhone will always appear as new photos. Please check the Settings > Photos. In the TRANSFER TO MAC OR PC section, tap Keep Originals to prevent the media from being converted to JPEG or H.264 when importing. Using HEIF or HEVC media on Apple devices - Apple Support

Does one of this apply? Have the duplicates been taken with the camera on the iPhone or saved from other sources? What is the file format of the duplicates? Have the duplicates been edited on the iPhone? The edited versions will also appear as new files.