Does changing and then changing back to the system photo library upload all the photos again resulting in duplicate copies on iCloud?

I uploaded all of my photos from one system photo library on my MacBook to iCloud. This is the photo library (let's call it the MacBook library) that I wanted to be my go forward default system photo library. I then downloaded a copy all of my my photos from my iMac to my MacBook (all photos still on iMac as backup, further backed up to a Carbonite account). I then changed my system photo library from the MacBook library to the recently downloaded iMac library so that I can upload all of the photos from the iMac library to iCloud. I can now see all of my photos from both libraries in iCloud which is great and exactly what I wanted.

Today, I changed the system photo library back to the iMac library (which as mentioned above, is the library I want to be the default system photo library going forward) and downloaded photos from my camera of a recent vacation to this library as a new event. I clicked on iCloud thinking only the new photos would upload to iCloud but iCloud calculated my library as having 15,000 photos and started the upload process which led me to believe that it was going to upload ALL of the photos from iMac library and not just the 100 or so photos that I recently added from my vacation. Obviously, that would consume a tremendous amount of iCloud space and result in duplicate copies of the iMac photos on iCloud which is not what I wanted.

Can someone please confirm that only the recently added vacation photos will upload to iCloud from the default iMac system photo library and not all 15,000 photos?

Thanks in advance.