A large import may leave several copies of the same photographs. Photos can identify duplicate groups, but a whole-library merge is a poor first experiment when the collection matters. Begin with a small group whose contents you recognise and preserve a current backup.

In a Mac version of Photos that offers Duplicates, open that collection in the sidebar; newer layouts place it under Utilities. Apple notes that analysis can take time, so an empty collection immediately after an import is not proof that every item is unique.

Inspect the proposed group

Open the photographs in a representative row and compare the visible subject, image quality and information important to you. Decide whether the group actually represents the copies you want to consolidate. A similar scene can still contain a different expression or moment.

If you use iCloud Shared Photo Library, check the library view. Apple directs users to Both Libraries when reviewing duplicates in that setup. Consider who else relies on the shared content before making changes to a large set.

Select the intended duplicate group and choose Merge Items, then read the confirmation count. Apple says one original remains and deleted duplicates move to Recently Deleted, where they can be recovered within 30 days.

Check the retained result before scaling up

Find the remaining image and open it at a useful size. Check the details that mattered in the comparison and confirm that you can still find it through the organisation you rely on. Record any unexpected result before touching another group.

Leave Recently Deleted intact while verifying the merge. Emptying it immediately removes a useful recovery opportunity and is not required to prove that the duplicate grouping worked.

Continue in manageable sets, separating uncertain groups for manual inspection. When finished, review several retained photographs and update the backup. The success measure is a cleaner collection containing the images you meant to keep, not the largest possible reduction in a counter.

References: Apple: removing Photos duplicates.