What makes a book unique?

I've run into this problem before, but I can't seem to find the solution this time. I'm breaking an omnibus into individual volumes, but I can't add them to my Kindle using calibre. I currently have four books, but as soon as I try to add a second one, the first one disappears.

The thing that makes it interesting is that if I just copy the files from the library folder to the document folder on the Kindle, the books work correctly, and now show up in calibre as being on the device, and match the ones in the library.

Any suggestions as to what I should be looking at?

Thanks in advance.