What happens when merging duplicate photos in Photos?

Exploring what you mean by "placed one photo in two files":

If you managed to import the same image twice (evading the duplicate check on import), the later scan can place them in Duplicates and one can be eliminated.

If those two copies of the same image had been placed into different Albums, Photos will make sure the surviving image is in both Albums at the end of the merge.

If you meant that you assigned one image to two Albums, Duplicates won't see that as a duplicate at all. All of the images in Albums are just pointers to the actual images, which are stored in the library.

When Duplicates is dealing with images of different sizes, qualities, and similar settings, it tries to do the "right thing" which may or may not be your idea of right. That's why you get the opportunity to review the sets of duplicates (triplicates, quadruplicates, etc.) before approving the merge. You might decline the automatic merge and handle it manually or just disallow it if the images are not truly duplicates.

Brian, Photos is listing two kinds of duplicates - identical image files and similar photos.

You can let Photos merge several groups of duplicates automatically by selecting several items at once, before clicking "merge".

• For similar photos the Photos.app will try to keep the photos with the best resolution from each group (the highest width and height in pixel), so you do not lose any quality.
• For identical photos the Photos.app will keep the version with the most metadata (titles, keywords, etc).

You might be satisfied with the result and save a lot of work, but I prefer to select the keepers myself. In some cases Photos cannot know which version we want to keep:

• The duplicates may have different or contradictory metadata, fpor example different locations, different capture dates, different titles or keywords. How is Photos supposed to know, which are correct?
• The duplicates may have different adjustments applied - which version is the better one?
• The duplicates may be similar photos taken in a quick succession. Which of them is the best shot? I prefer to be the judge of that.
• The duplicates may have the same pixel size but a different file size and compression? Photos will pick the file with the smaller file size. We might lose quality.

If you let photos do the merge automatically, you will find the discarded photos in the Recently Deleted album and can put the photo back.

Most of the potential Duplicates shown by Photos are duplicates I created intentionally and want to keep

• Live Photos with different animation effects
• Edited versions of the same photos cropped differently or adjusted differently, for example a Black and White version in addition to the color version
• Photos I need twice in project, for example in a slideshow

We cannot mark the duplicates that we want to keep, we will have to be careful not to delete them, when we select several groups of duplicates together for merging.

A Duplicate means that there are actually two copies (or very similar files) in the Library, no matter how many Albums either may point to either or whether they are in a common Album.

Thanks for the very helpful advice. I appear to have lots of duplicate photos in my Library, sometimes up to 5 copies. I think this may be the result of simply transferring the Library from computer to computer over 20 years. I am trying to eliminate duplicates to save space and improve speed. However, I am spending hours having to review each pair (or more) and approve each individual merger. Can the software do this automatically? As I have more than 30,000 pictures, doing it manually could take the rest of my life!