It seems I've inadvertently moved files to my desktop, and in trying to get them back in the library, have duplicated many photos.
Do I understand you correctly? You accidentally moved original (master) image files and now you cannot edit some of your images? Did you remove managed originals from your Aperture library package or move referenced originals?
If the files that you moved are referenced originals, then your best option would be to reconnect the original files to your versions instead of reimporting.
But if you moved the originals from there masters folder inside the Aperture library package, then it is not possible to reconnect them. You will have to reimport them (as you seem to have done already) and to delete the versions, that now have no original masters.
What have I done? Without deleting the master, can I delete a version that appears in the browser, but has no metadata attached (so I can no longer edit it)?
If you cannot adjust an image, this is due to a missing original master image file, not to missing metadata. You receive the warning, because you used this version in some products: albums, books, etc. Deleting this version will not delete any master, since the master is lost already.
Before we can advise you on how to proceed from here, we need to know, if your masters have been referenced or managed.
Pleas post back; then we can discuss how to reconnect your masters or how to reimport.
It is difficult to answer, as I believe I have done a combination of deleting masters, moving them to the desktop, (where they became referenced originals), relocating the masters back into the library (so they would become managed originals). In the process, I lost some masters, or lost the connection between a version and the master. I do not know at what point this happened, and it is not the same project to project, nor is it consistent within a project (I shudder as I write this).
For about 90% of the photos, I have disk backups, and have reimported them in (different) projects. The ones that I've edited? I've given up trying to recover/save my editing. I've spend considerable time on photos in some projects, but I am trying not to think about that right now.
So, now, I have to clean up the mess I've made. Serves me right for not going through the manual chapter by chapter beforehand.
My question is regarding photos with lost originals (image not found; not able to edit). I did attempt to just delete these versions, but received the message that these "originals" were in other albums, and would be deleted if I deleted the version. If I accidently deleted the master, why would I get that message? I just wanted to know I can delete those versions without causing myself more headaches (i.e., deleting the new master I've reimported).
Error messages are not always perfect. That is the standard warning you get, when you are about to delete a version that is included in some book, album, screensaver,...
It does not necessarily mean, that the master is still existing.
When trying to delete one of these versions, I get the warning you received, because these images are part of an album, but the images no longer have original images, as you can clearly tell by the badges in the lower right corner.
These versions you can delete, if they are not referenced and you do nor want to reconnect them to their originals to save the edits.
You may want to create a smart album containing all images with missing masters/originals, so you can flag them and spot them easily. Define a smart album based on a rule with the file status "missing":
Then you can flag or delete all version at once, that have missing originals.