Photos not recognizing already imported photos

manu_18 wrote:

through WhatsApp

My photo library is on an external drive

Social media sites like WhatsApp, Facebook etc strip all metadata from images. Then all image dates are usually based on very fragile filesystem file creation dates instead more robust internal EXIF dates. So apparently Photos can't tell that those images are already imported and treats them as new.

One way to avoid manually guess and insert proper dates might be to route the images via iCloud Photos from the iPhone Photos. Then the dates would be at least the date when that image arrived from WhatsApp to the iPhone (but not the correct datetime when the image was actually shot...).

Make sure the external drive is formatted as APFS or MacOS Extended because non-Mac formats like exFAT are not supported and might corrupt the library and do not work properly anyway.

Have you changed any Settings on the iPhone in the Photos > Transfer to Mac or PC section?

If you have previously been using "Most Compatible" and converted HEIC files to JPEG on import, and are now importing without converting the media, Photos will no longer be able to recognize the photos as already imported, because they are now in a different format.

Matti Haveri wrote: Make sure the external drive is formatted as APFS or MacOS Extended because non-Mac formats like exFAT are not supported and might corrupt the library and do not work properly anyway.

Adding: Even though there were warnings from Apple, before Ventura (I think) Mac OSs allowed Photos Libraries to be run from ExFAT drives. We here heard lots of horror stories about Libraries going along smoothly, apparently working quite well, and then suddenly refusing to cooperate, losing lots of information and allowing limited recovery. Since Ventura, Photos refuses to open Libraries on improperly formatted drives. So now we see "Why do I get this 'cannot be opened'" questions.

So Matti's warning is especially important since you are running an older OS. If your Library is not on a Mac formatted Library, you should stop running it immediately.  To avoid damaging the Photos Library an external drive must be formatted in either APFS format or Mac OS Extended (Journaled) format. The drive must be connected directly to the Mac by cable, not networked, clouded, NASed, etc. Additionally, the drive can not have had Time Machine on it since it was formatted. There have been so many problems with using incompatible drives that the newest macOSs won't even allow a Library on a non-Mac formatted drive to open, since there is a chance of damaging the Photos database. See this:

Move your Photos library to save space on your Mac - Apple Support

APFS has been standard since High Sierra.

One problem is, that the Photos.app is now assigning internally new, unique filenames to downloaded photos. You mentioned that the shared photos are now having different filenames.

I am afraid you will have to remove either the renamed versions from your iPhone or the already imported images from your Mac to fix this.

The Photos Library must not be on the same volume or partition as the Time Machine backup. Volumes containing a Time Machine backup are protected and the Photos.app may have problems with the file ownership when trying to work with Photos Library. Do not put a Photos Library on a volume, where you cannot set the "Ignore Ownership on this Volume" flag as described here: Move your Photos library to save space on your Mac - Apple Support

You could put the Photos Library onto the same drive as your Time Machine backup, if you create separate volumes on the drive. But you would still have the problem, that the Time Machine backup of your library will be on the same drive as the Photos Library and you cannot recover the library from the backup, if the drive should fail. You really need two separate drives, one for the Photos Library and and one for the Time Machine backup, if you value your photos.