Photos in double in PHOTOS library that cannot be deleted on iPhone. Origin of problem = aborted synchronisation with iMac.

The easiest way would be to enable iCloud Photos Library - just for a short time. When we turn on on iCloud Photos in the Settings > AppleID > iCloud > Photos, all synced photos will be erased automatically, because we cannot use the manual syncing at the same time as iCloud Photos.

When you turn on iCloud Photos, do not enable the "Optimize iPhone Storage" option, otherwise some the photos you want to keep might be removed from the iPhone.

Once the synced duplicates have been erased, you can turn off iCloud Photos again, if you do not want to switch to from manual syncing to iCloud Photos.

Pictures synced from a Mac to Photos by cable (instead of by iCloud) don't really belong to Photos, and they aren't in the Photos Library-- so Photos can show them, but it can't delete them. The solution is typically to re-sync with the computer using the cable. That should work in spite of the interrupted transfer, I think.

You may find that it's easier using the Image Capture app that's in the Applications folder.

Let us know how it goes...

Soyajoy wrote: ... but 'Synchronizing' to me, means that the 2 sides will do/be the same (identical)... no?

What about the Image Capture app that's in the Applications folder, that you talk about?

If you use iCloud Photos to synchronize your Mac and your iPhone, then "synchronize" will be exactly what you think it should be. Any pictures on either device that aren't in iCloud Photos Library will be copied there. Any pictures that are in Cloud will be copied to any device that doesn't have them. Photos will keep the pictures in the Mac Library, in the iPhone Library, and in the iCloud Library exactly the same. Photos tries to catch and eliminate duplicates during the import process. You may choose, on an iPhone with smaller memory, for instance, to keep only smaller images, and Photos will reach out to iCloud and grab full sized versions for zooming, editing, printing, and so on, as needed.

iCloud costs $3/month for 200 GB (well, since you write "synchronise rather than synchronize, I'm guessing you might want it in £ or €.) And if you hate it, you could do it for a month, get everything synchronized, and then drop iCloud. But really, it's cool that it's all automatic and pretty cheap.

The term "Synchronize" is also used for the process of transferring images with a cable, but it's just not the same. The actual procedure depends on the OS. Image Capture is an Apple app that's been around a long time, and it seems robust and reliable at finding files in different drives. Sometime it works better than hoping the automatic stuff works. I use Image Capture to operate my flat bed scanner, for instance, because the maker's software always seems full of foolishness about stuff I don't want, and Image Capture just works. (If the maker allows...)

If you want to use iCloud, you should ask us about Optimizing Storage and backups.

Great! The Community here can help if you have more questions...