iCloud Photos on two duplicate iPhones - duplicate photos?

I've just set up a replacement iPhone XS Max and used the Quick Start function to copy everything from my old phone which was damaged, albeit still usable, to my new one. So now I have two identical phones with identical data and photos on them.

I have iCloud Photo turned on on both phones. My damaged phone is only about half way through syncing my 20,000 photos to iCloud (I only enabled it a few days ago so not all yet uploaded).

My new phone with all the same photos is now starting to sync those photos to iCloud as well.

Question, will this cause duplicate photos on iCloud, or does Apple de-duplicate them?

If iCloud duplicates them because it sees both instances of each photo as a separate file, can I remove one set by removing my original phone from iCloud? Or do I need to remove all my devices from iCloud, delete all three photos on iCloud, and then turn it on again to reset everything?

Any thoughts, help, or suggestions will be greatly appreciated. Thanks 😃👍