Well, after an artwork re-scan, all the duplicate images seem to have vanished, leaving all albums with artwork to 'link to an organised copy'.
I had left the config setting to 'save as folder.jpg', so, when I saved artwork that didn't have an image, it left it as 'save to...'. Replacing existing artwork did a 'link to...'.
Changing the config setting to 'link to...' does as expected, and saves using 'link to...'.
I think I've got to the bottom of what controls I should use, but I'm still not sure how I managed to get all those duplicates. Maybe I must have had both 'save to...' and 'link to...' options set when I did the first import, and setting just one option and re-scanning artwork has fixed the dual image problem, which, in turn, has allowed the file to be saved. Though I'm not sure why it wasn't able to save the file, and why it touched the file.
MM allows you to add a number of artworks. My original ripping and scanning process created 'folder.jpg' and 'rear_cover.jpg' images in the album folder (with master artwork in an Artwork subdir, and rip report and track listings squirrelled away into a Riplog folder), and MM found and added both of those automatically during import, and made folder.jpg the default image. I never really found a way to select, oher than editing to change priority.
I may create a new, small test library to investigate the import behaviour.
Until then, thanks again for your assistance.