Mac storage basics · Photos, music and video
How do I stop Photos importing the same pictures twice?
Short answer: Use Import All New Items when importing, and decline duplicates when Photos asks. For ones already in the library, use Photos' Duplicates album under Utilities.
When you import from a camera, card or iPhone, Photos shows which items are already in your library. Use Import All New Items (rather than importing everything), and if Photos asks about duplicates during an import, choose not to import them.
For files dragged in from Finder, Photos warns when it spots an exact duplicate. Existing duplicates in the library can be found in the Duplicates album that Photos shows under Utilities in recent macOS versions; Storage Studio's own duplicate finder is covered in Duplicate files.