Delete duplicate photos but not those already in albums

Or dot the other way around - tag all photos in an album with the name of the album as a keyword.

Then you can see, which photo has been added to which album, when you look at the Info for the Photo.

When deleting duplicates, it is essential to keep the better version - the one with the best resolution, or the most metadata tags. If you find, that the better version is not in any album, but the lesser version is, you will still want to keep the better version and then add it to the correct alum, while deleting the other photo. Or the version not in any album may have been tagged with a title, caption, and keywords, but the other version has not, and in that case you need to transfer the title and caption to the photo in the album.

Hi

Try this:

Tag all your duplicates with a keyword (eg "duplicate")

then create a smart album with the conitions:

Keyword is duplicate

and

Album is not any.

Like this (though I've used bird for the keyword)

Note that if you have dupicates that are not in any album, then you will see both (or more) copies in the smart album. Then you will have to be carful not to delete them all.

So a better way may be to create a smart album with the condition "Album is any".

Select all the images in that smart album, and give them the keyword "in album". Then when you are looking at duplicates with the info pane open, you can see which ones are in an album.