IDPKT wrote:
... (appreciate this is a link not the actual photo) to a Shared Album it wouldn't copy duplicate links ...
Shared Albums are different from standard albums. Shared albums indeed are holding duplicates and not links. The reason is, that Photos is rending special, new versions of the photos you are sharing, optimised for sharing. When you share a photo, Photos is downsizing the image to 2048 pixels at the longest edge and removing most of the metadata. This way we will not dump large images files onto the devices of the subscribers and are not accidentally giving away more information than we want to share.
As far as I can tell, Photos has always added the same photo to the shared albums again, when we shared it again. I have used this frequently to share the same photo again, after I added new adjustment. This is necessary, because the shared photo is independent of the photo we shared and when we add adjustments in our library, the adjustments will not be synced to the shared version. I tested again on my older Mac with macOS Mojave. When I share there a photo ten times in a shared album, it appears ten times, even on the older system.
To find the missing photos quickly, that you need to add, you could try to sort the shared album by the date, also the local album on your Mac. This should make it easier to spot the missing photos.
