iCloud & iCloud Photos · iCloud Photos and the Photos library
How do I find duplicate photos in my Photos library?
Short answer: Use the Duplicates album in Photos' sidebar to merge duplicate photos; the change syncs through iCloud Photos. Never edit inside the Photos library in Finder. Storage Studio's Duplicates window handles ordinary files elsewhere.
Use Photos' own Duplicates album. In recent versions of macOS, Photos lists duplicates in the sidebar, and you can merge them there — Photos keeps the best version and combines their details, and the change syncs through iCloud Photos.
Don't delete files from inside the Photos library in Finder; the library is a managed package and editing it from outside can damage it.
Storage Studio's Duplicates window is for ordinary files — in your Downloads, Documents, Desktop, Movies and Pictures folders, iCloud Drive, and connected cloud accounts. It's ideal for stray photo exports and copied folders outside the Photos library. See duplicates in iCloud Drive.