double photo download

Before importing into iPhoto unplug the iPhone and plug it back in. That should clear the duplicates and provide a clean import.

An add-on to Old Toad's advice:

When I download my iPhone 6 pictures to my computer (iPhoto updated since purchased in '09), I end up with 2 copies of each photo. How do I make this stop happening? Thank you.

If you have Photo Stream enabled, the reason for the duplicates is probably, that iPhoto is importing from the Photo Stream as well as from the Camera Roll. In that case it would help to launch the Camera.app on the iPhone and keep it open, while you download the photos. My Photo Stream does not upload, while the Camera.app is open on the iPhone.

If you are shooting HDR on your iPhone it is normal to download all photos twice. Then you are downloading the normal version in addition to the HRD version. This can be disabled in the settings, but I would not do it, because it is better to compare both versions on the Mac, before you decide which version to keep.