Mac Photo editing. Duplicate photo finders Gemini 2 and others.

The Photos.app has its own built in duplicate detector, and it would be the safest tool to remove duplicates and ensure that the remaining copy will remain in any album we assigned all versions to.

Gemini 2 is a tool to remove duplicate files, not a dedicated duplicate cleaner for images and videos stored in a Photos Library, possibly in iCloud. I prefer duplicate cleaners that have been designed to work with the Photos.app and iCloud Photos, for example Power Photos.

Agreeing with léonie -- I have used Gemini 2 for pictures outside of Photos when I had a massive number of picture files recovered from old backups (like on CDs floppies!) But that was before they charged by the year.

As léonie implied, apps like that have been know to corrupt a Photos Library. While some such apps may work, there is quite a risk. Instead, many of us use the trusted 3rd party app PowerPhotos ($30.) PowerPhotos gives extensive of control in filtering duplicates, and it does lots of other helpful stuff. (I use it for the other stuff more than for duplicates.) I have also used PhotoSweeper ($15) which is cheaper but does a whole lot less.

In general, try to stay away from apps that are free to roam around through all your files looking for things to change. Mac "Cleaners" are to be avoided.

Oh-- make a backup of your Photos Library (indeed, you system) before doing this stuff!

I wonder, why Apple has removed this very useful knowledge base article. Now I can only find it on the internet archive "wayback machine". https://web.archive.org/web/20190408220612/https://support.apple.com/en-us/HT204968

"Using third-party apps to remove duplicate photos might damage your Photos for macOS library

Apple recommends that you avoid using certain third-party apps that can damage your photo library on your Mac.

Some third-party apps for your Mac that claim to clean, shrink, de-clutter, or remove duplicates from Photos can instead damage your photo library or delete pictures and related data.

You can avoid damage to your library by organizing and managing your pictures from within Photos. 

Information about products not manufactured by Apple, or independent websites not controlled or tested by Apple, is provided without recommendation or endorsement. Apple assumes no responsibility with regard to the selection, performance, or use of third-party websites or products. Apple makes no representations regarding third-party website accuracy or reliability. Risks are inherent in the use of the Internet. Contact the vendor for additional information. Other company and product names may be trademarks of their respective owners.

Published Date: September 20, 2016"

It is even riskier now to use cleaning apps on a Photos Library than before, if we are using iCloud Photos.