how do i delete duplicate photos on my i Mac

Are you, in fact, using macOS 10.11  El Capitan on your Mac, as your signature says? In newer OSs, 13 and later, Photos has a built in Duplicate finder. But for El Capitan, you would need a 3rd party app. The favorite here is the trusted 3rd party app PowerPhotos ($30), which has lots of useful function in addition. Another is PhotoSweeper ($10) which is cheaper but does less.

It's troubling that you keep getting duplicates. What OS are you running on your iPhone? Newer OSs may use HEIC files instead of jepgs, for instance, and your Mac may be seeing newly rendered pictures as different images. You may want to check the Formats Settings in the iPhone Camera-- Most Compatible might be best. And in Settings for Photos, Transfer might be better set to Automatic. But then again, if you change these now, you might get a whole slew of new duplicates. You could try it and see.

Another option is to give up connecting by cable and use iCloud Photos. It costs $3/month for 200GB, and it synchronizes the iPhone and Mac Photos Libraries so that they always have exactly the same pictures. It's different, but pretty painless.

Let us know how it goes....

I get the same with my iPhone 12 Mini and Photos V 8.0 library.

My workaround is to use Image Capture. There you can select the photos to import and get only the new photos.

As for existing duplicates and if you're still running Big Sur there are a couple of apps that find and identify duplicates so you can make the decision on which to delete.

You want an app that will identify the potential duplicates, put them in an album or mark them with a keyword for easy retrieval and deletion by you. You don't want one that does the deletion itself for obvious reasons. 

I've run tests on the these two apps with the following results and found them to be safe to use:

PowerPhotos - $29.95  

PowerPhotos is the iPhoto Library Manager version for Photos and is very powerful. Although more expensive I would recommend it as it has more capabilities than the others like the capability to merge Photos libraries or copy photos, both original and edited versions, along with their metadata between libraries.

PhotoSweeper - $9.99 - Demo version available.

PhotoSweeper compares bitmaps and/or histograms so it can detect duplicate images even if they have different file sizes, file names, image sizes and capture dates.