iBooks import: "Some of these books are already in your library" Replace or Cancel

I'm trying to determine if I've successfully imported a large directory of PDFs into iBooks (10GB).

If I try to import all of them again, I get the expected duplicate warning:

Some of these books are already in your library.

Do you want to replace them with the ones you’re adding? The rest will be added automatically.

Cancel Replace

Now I don't want to spend time replacing what I already have, as it will re-upload them to iCloud. But I do want to import/upload anything that iBooks hasn't already got. I don't seem to have a choice here - all I can do is click Replace and re-upload 10GB, or click Cancel.

Anyone know a better way to do this? Is there an API or something that I can call with the list of PDF filenames to see which ones iBooks does/does-not know about, so I can filter the list down to just what is missing and import those?