I am not sure how this has happened, but my catalog has a number of photos with icons in the top right corner which I took to mean stacks. But in using the Find/stacks command, I found I actually have none. They are version sets. Whatever that is. When I use Ctrl D to change the grid, I see a small arrow on the right hand side of the picture, about midway down. Clicking on the very first one on the list, it appears to be the same photo, with a different orientation.
1) is there some reason why I would need both orientations?
2) are version sets limited to two versions for each photo? If not, how do I tell which photos are "versions" of the main one? (In my grid, Version set A seems to be parked right next to Version set B, so I am not sure where one stops and the other begins)
3) if the photos are always identical (are they?), is there some single command that will remove the unnecessary versions? Will this affect the thumbnail links to the "main" or principal version?
A version set is a group/stack of photos containing edited versions of the same photo. You have to take some action to create a version set, even if it was simply rotating the image and saving it (with a different file name). When you edit a file and save it, there is an option to include it in the Organizer and another option to save it in a version set.
A regular stack can contain totally different photos. It is just a way of grouping your images in one place. For example, If you took a photo of a child on each birthday, you could group them all together in a stack of birthday photos.
You can have a regular stack that contains version sets of the same photo. If you shoot raw and jpeg files, there is an option in the Adobe Photo Downloader to automatically stack the two formats so only one of the "identical" images is displayed in the Organizer grid. Normally the raw photo will appear on top. But you can always change the order of a stack or version set to change the top photo in the stack.
To answer your specific questions:
1. That is entirely up to you. But before giving you definitive advice, I would want more information about how you ended up with the two photos in different orientations, the format of the files and specifically whether the photos are HEIC photos from an iPhone or iPad. For some reason, Elements cannot handle different orientatations of HEIC photos. If you need to rotate a HEIC photo, it cannot be saved in the HEIC format and needs to be saved as a jpeg/tiff photo. This may be the reason why you have ended up with identical photos but in different orientations. They are not exactly identical since they will be in different formats.
2. There is no limit on the number of versions you can have in a version set. I am not exactly clear about the rest of your question. Perhaps if you gave us a screenshot, we can be more helpful.
3. Version sets are never "identical." At a minimum, they will have different file names. Each version will also have whatever edits you have made in the Editor. If you only want one of the photos in your catalog, you can select the other(s) and delete them (from the catalog). You are also given the option to delete the selected file from your hard drive. Deleting one of the photos in a version set/stack has no effect on any other photo in the set or stack. BUT, if you delete the top photo in a stack/set you will be presented with an option to delete all files within the stack/set (and to delete those files from the hard drive). Be careful about which options you choose.
Thanks. I figured out (2) by using the zoom function on the bottom right and, after clicking on the right arrows beside photos to unpack the related secondary version, I was able to put the photos (the main versions, that is) each on one line, and delete the one with the wrong orientation.
In reply to (1), I did not (actively, anyway) save a new version of anything. I rotated the photo and moved on, not realizing that PSE was doubling them up.
As you say, I have been careful to delete the ones with the wrong orientation from the catalog, not the source file per se, to avoid a possible missing file issue down the road. But if you're right, that some of my source files themselves now have duplicates (from my changing the orientation), I should go back and delete those from the source files. No need to waste space.