ISBN not unique?

[Clouddancer]

I've got a comic book that was published and sold in the '80s as two volumes (part 1 and 2), and both parts have the same ISBN - I double and triple checked this.
When I tried to add them manually, I got the "ISBN has to be unique" error. So far I added part 1 with ISBN, and part 2 without, but is there any other way to deal with such cases?

[[deleted user]]

Hi. Allow me to give an example:
http://www.goodreads.com/book/show/16...
this work, a 2-volumes lexicon, has 3 distinct ISBNs
- one ISBN for the whole set, and
- two ISBNs for each volumes.
Ideally, it's supposed to be like that.
My suggestion is as with Abigail, if you can't find the ISBNs of the individual volumes and just have one ISBN for the whole set, just make one entry with an explanation that's it's a two-volumes work.