All, I spoke to Apple support today (3 Nov 2017) and they confirmed that as of today Photos App has no way to identify or find duplicate photos in your photo library or icloud library. Please log a "Photos Feature Request" or "Photos Enhancement Request" using Apple "Photos" app feedback form below to help resolve this problem
Apple support "photos" specialist confirmed following 3 Nov 2017:
1) The new Photos app and old iphoto app had a way to search for duplicates on import this does not work once the duplicate photos are imported into library (by error or otherwise)
2) Apple photos has no way to search for duplicate photos, they suggested that users "manually search for duplicate photos".
3) Apple support do not recommend third party apps to remove photos as they can damage the library.
4) They acknowledged this approach is less than ideal when you have 20,000+ photos (or in my case 80,000 + photos) and suggested that all users who have concerns should log a new feature request using apple feedback form (URL below for IOS and OS Mac Photos feedback form).
Please log a "Photos Feature Request" or "Photos Enhancement Request" using Apple "Photos" app feedback form below to help resolve this problem
Submit Enhancement Request using Apple IOS "Photos" app feedback form
Submit Enhancement Request using Apple Mac OS "Photos" app feedback form
Cheers MR HK