Some advice about ISBN conflict, please?

[Phil]

I have in my hand a copy of the combined edition of The Midnight Folk and Box Of Delights which has the ISBN 0-7497-4872-9 on both the back cover and the copyright page.

However here on GR that ISBN is attached to a hardcover edition of The Midnight Folk. I cannot find any copy of the combined edition on GR anywhere.

What is the best way to add this combined edition and attribute that ISBN to it without wrecking the edition which is currently (incorrectly as far as I can tell) attached to it?

[Amara]

It appears that record originally was listed as The Midnight Folk and Box of Delights until someone changed it several months ago. I've reverted the changes and added a note explaining what to do if a librarian discovers that this is actually a case of reused ISBNs. (Which I'll do myself if I can find any source to confirm.)

ETA: And if you can supply the correct cover, that'd be much appreciated.