Q: In Photos Library, duplicate photos of different resolution

From what you have said it's not really clear where the photos are from or what they are. In iPhoto, photos that were imported from photo stream should have been imported as full resolution images on your Mac, it's mobile devices that see a reduced resolution version. So yes, importing on a Mac should result in the photo stream keyword but they should be full resolution images.

Do these photos appear in your photo stream album on your Mac. Do you see the photos on your mobile devices and if so where.

If you ever import a library you create a mess of different version of the photos which is what you state you have done - his creates many more problems than just the multiple versions of photos

You NEVER import a library - to move to Photos yo migrate the library - drag the iPhoto library to the Photos icon in the dock and it will migrate with no duplicates   --  How Photos handles content and metadata from iPhoto and Aperture - Apple Support

The only real solution is to migrate and start again with a good library

As to deleting the extras  you can but only do it using Photos - do not ever change anything using the finder

LN

And could you also explain these in more detail?

I recently imported my iPhotos library into Photos,

How did you do that? Did you use "File > Import" to import the photos from the library or did you let Photos open and migrate the library (you migrate an iPhoto Library by dragging it onto the Photos icon in the Dock or holding down the options key while launching Photos to bring up the Library Chooser panel, see: Updating from iPhoto to Photos for OS X - Apple Support)

If you really imported the the photos from iPhoto to Photos you will have many duplicates, because the thumbnails and previews will also have been imported.

I am seeing duplicate photos, with one photo at higher resolution, and the other at lower resolution, with the automatic tag "photo stream."

Did you import photos from your iPhone by connecting the iPhone to USB? When I did that, I imported plenty of lower resolution photos with weird filenames, apparently random sequences of letters.  What are the filenames of your duplicates like?

But I wonder why it would import a lower resolution version, rather than identifying the photo as a duplicate.

Photos does only recognize exact duplicates as duplicates.  The file attributes need to be the same. If your iPhone saved low resolution versions to the Camera Roll, they will have a different file size and a different date modified, or even different filenames. They are no duplicates for Photos.

You can delete these low resolution versions, if you are sure you are keeping a better version.

If there are too many of them, use a duplicate finder tool like Photo Sweeper. It can recognize lower resolution duplicates by comparing the histograms or bitmaps.

Power Photos can also detect them, if at least the filename remained the same.

BTW. The low resolution versions I imported from the iPad by USB, were photos transferred to the iPad by Shared Albums. The Shared albums transfer the photos in a reduced size, even to the Mac.

On my iPhone they appear in a folder called "Events from my Mac."  These are pictures from my travels, and I may have indicated in iTunes that I wanted them to remain on my phone.  Does that help?  Thanks for helping!

Yes that makes sense. However as you move over to Photos you are going to need to redefine what you sync to your Mac since the source has or will soon change. I suspect you can delete those low resolution images without affecting anything on the phone, however I'm wondering if you might have initially created those low resolution versions specifically to sync to your phone for space saving purposes.

I assume that I no longer need to connect to iTunes to download photos; that it will be done wirelessly?

Yes, you should now be able to do that with photos stream only.

But it appears that My Photo Stream photos are only kept for a limited time?

That's correct, 30 days. Photo stream photos should be automatically imported to your library on your Mac, so when they do disappear from the photo stream album you will still have them in the library. You may wish to open Photos periodically to check this has occurred.

And how do I assure that I get the highest resolution photo to download to my Mac?

Full resolution images are always sent to your macs photo stream album, you shouldn't have to worry about this. Clearly the ones you spoke about earlier have come from somewhere else, which is why I was asking about them