Shared Photo Albums

Hi, I use Apple photos app to archive our family's photo collection that I have scanned into the computer over time. I tag images for the family collection with a keyword and have a smart album setup so that they are all in one place. I also have a Shared album that gives other family members access to the photos.

My issue is that is fine if I do a batch of photos today and drag them to the shared album right away, they land at the bottom of the smart album and can easily be dragged across and added to the shared album. However odd photos that I have added and not dragged across at that time could be anywhere within the smart album and therefore the easiest way to ensure all photos from the smart album are in the shared one is to select all items in the smart album and drag them to the shared one. My question to you is does this only add the 'missing' photos to the shared album or does it duplicate existing ones too as this is what I want to avoid.

Currently my smart album has 3,745 photos in it and the shared album has 3,605 photos in it.

Any help / advice would be gratefully received. Thank you.