Thanks RoderickGI,
"If you have two entries in MC, highlight the bad one, press delete, and you should get three options on what MC should do in a popup dialogue.
Select "Remove from Media Library". That should leave the good entry in place, and remove the bad entry, but leave the actual files in place on your disk."
From the phrasing "Remove from Media Library" I interpreted that to mean it would remove the ripped bluray files from the drive (using JRiver to rip, not an external program). The original rip was suspected to be bad, so when I re-ripped it, it found the original rip and asked if I wanted to keep it- I replied 'No', and it was removed and re-written, so both the original entry and the re-rip point to the same location.
"There are some other considerations, such as multiple entries in the "CD Database", whether you have "Fix broken Links" set to No under Auto Import, and so on, but the above should fix it without further action, I think.
Or have you customised MC settings a lot, which can, of course, change behaviour?"
I have changed very little from the defaults in MC other than settings for playout (bitstream via HDMI, enable server-client, and DLNA); this is my JRiver server, and all media paths have been edited to UNC paths- don't know if that alters your advice. Settings related to myltiple entries in the "CD Database", "Fix broken Links" under Auto Import, etc. should all be set to JRiver defaults. Will back up the ripped bluray files before deleting the extra library entry JIC.
Cheers!