Editing copies of a photo on MacBook Pro

When you said you duplicated the photo, did you actually use the Duplicate function (in the Image menu or the control-click menu)? That would work as you expect.

It sounds more like you used only Copy and Paste into a different Album without the true Duplicate function.

Photos is what they call a lossless editor. That is, it keeps the original intact and tracks changes. When you use Duplicate, it builds a separate set of changes for the same original (with the same filename, true). You can work on the two change sets independently, as if they were two different photos.

When you add a photo to an Album, you are just getting a pointer to the original photo and its change list. Editing the photo there changes it in both locations. Pasting it back in the original Album has no effect, as the photo is already a member of that Album and already has the same change set.

When you copy and paste a photo, you import the preview of the edited version as a new file. This is wasteful. Do not use copy and paste to add photos to albums. Just add the photo to the album with the "Add to album" command as Mark recommended or drag and drop the photo to the album in the sidebar. This will also add it to the album, without importing a new file.

When I want to have two differently edited photos in an album, I drag the photo to the album in the sidebar and then view the added photo in the album and duplicate it while viewing the album. This will add the duplicate to the album and you can edit the duplicate independent of the other version of the photo. For example, I wanted a black and white version of the photo, where only the blossom had color: