Delete Duplicate Photos & Date

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Like what device are you on? Model and year.

What system version?

If a Mac where is the library located? If on an external drive how is the drive formatted and connected to the Mac.

How much free space is on the device?

Apple does not describe how conflicting metadata will be resolved, when we are merging duplicates, that are not exact duplicates but similar versions with different metadata.

See: Remove duplicate photos and videos on Mac - Apple Support

When we try to merge duplicate photos, that are similar but not exact identical copies, we will see a warning:

This does also not tell us what will happen and how the metadata will be merged.

We have to check manually, which version has been kept and which has been deleted.

I am adding all duplicate photos to an album, before I let photos merge the duplicates. It the saved phot has the wrong date or missing keywords, I restore the the deleted version from Recently deleted.

Frankly, I am using the Duplicates album only find potential duplicates, but I rarely use the automatic merging. I am checking the titles, captions, locations, dates and keywords manually, and copying the correct data to the "keeper", then delete the redundant version.

Photos has no way to know, which date or location are the correct on, if they differ for two duplicates.

I want to second léonie's point-- I never let Photos automatically "merge" duplicates. If I use "Merge," I do it after I've made the keywords, captions, and locations match. Also, resolution and file size are two independent (but connected) variables, and one picture might have higher resolution but be more compressed. In many cases, you just have to look at them and see which one looks better. If dates are different, I just have to try to judge which is more accurate, and then use that date to adjust the date of the one I thought was better.

The best part of "merge" is that, if the "duplicates" are in different albums, it puts the surviver into all the albums of the deleted version. But, because of the above limitations, it's really hard to use "merge," sometimes. I use "File>Show In Album" to get the chosen "better" one into all the right albums, but it's tough to be sure I've gotten it right.

The best thing to say about merging duplicates is, "Use caution!"