Q: What happens if I turn off my iCloud Photos Library and want to turn it back on later?

With an iPhone 6s running iOS 10.0.2 that i've just figured out I probably turned off the icloud Photos library back in March 2016.  when I go to turn this back on in my iPhone under settings>Photos&Camera>iCloud Photo Libary my phone is saying its uploading 4300 photos. But currently in my iCloud Photos Library there are 3000 photos.  Which after checking I see all 3000 photos exist on my iPhone 6s.  SO my question is here if I turn the iCloud Photo Library back on will all 4300 photos be added to my iCloud Photos Library?  Leaving me with 3000 duplicate images in my iCloud Photo Library? Or does the iCloud Photo Library recognize duplicates and only import the 1300?  If not I obviously do not what to import duplicates, so how would I import only the delta, or the 1300 images, to the iCloud Photo Library?