Mac Photos keeps import the same photos

There are two common possible reasons for this:

• iCloud: If iCloud Photos is enabled on your iPhone or My Photo stream, you may continually be seeing local copies of the cloud photos downloaded from iCloud as "new" photos. Using iCloud Photos or My Photo Stream will make the duplicate detection difficult, as the downloads are new files.
• Format conversion to JPEG: if you are taking the photos as HEIC files on the iPhone, but are converting them automatically to a more compatible format, when you import them to the computer, the original HEIC files on the iPhone will always appear as new photos, because you are not importing them, only converted versions. Avoid a format conversion when importing from the iPhone.

Some users reported, that whatsup photos also appeared as duplicates. I cannot test this, as I am not using WhatsApp.

To avoid duplicates on import, delete the photos from the iPhone, that you already saved on the computer. Or, if you are using iCloud on the iPhone, use it also on the computer and avoid the manual download from the iPhone.

Since a couple of months I have the same problem when importing photos from my iPhone SE 2nd generation (iOS 14.8) to Photos 6.0 (361.0.100) on my MacOS Big Sur 11.6.1.

I have tried turning on/off automatic conversion when transferring pictures in my iPhone settings, but doesn't make any difference. I am not using iCloud photos or offloading photos to iCloud.

It just seem that photos or videos that I have saved or created in Instagram et. al. always get duplicated. In the past, a few years ago, I had a similar problem with all content saved from Snapchat but now with Instagram et. al. that I use more frequently it is becoming really annoying.

Deleting the original from my iPhone or using Duplicate remove programs are not an option. There is some kind of bug and I hope that Apple pick this up and investigate the cause of it.

The duplicate detection in Photos for Mac can only detect exactly identical files. If some syncing service, like Instagram, keeps downloading similar photos of the same file, they will appear over and over again as new files. In that case you have to select the really new files manually on import and not trust the automatically shown new photos.

I had this issue too with Photos 6.x in Big Sur (11.6), but I've now upgraded my Mac to Monterey (12.x) and through that got Photos 7.x. Since then I no longer get duplicates when importing from my iPhone. Same Mac computer and iPhone as before, just a newer Photos version since I upgraded the OS on my Mac.