The short answer is that it’s not implemented yet, but hopefully soon (in a release or two, I hope).
The long answer is that it’s harder than it looks if you are using the Photo Library. Actually, it’s impossible to fully implement Duplicate in the Photo Library. That’s because Apple does not permit third party apps to read/write keywords, titles, descriptions, etc. As a result, I cannot make a new image in the library that includes the titles, descriptions, or keywords you may have added in Photos. I can copy Nitro data, but not Photos data. Argh. So, when I make the Duplicate feature in Nitro, any duplicate made in the Photo Library will be missing that data.
In the intervening time:
1) Finder: Use Duplicate in the Finder – if your disk uses APFS, then the duplicate will not use more disk space
2) Photos: Use Duplicate in Photos – they can make a true copy.