I am converting some AZW3 books (mostly Amazon freebies acquired last year) to ePub. 
Occasionally, the automatic split of chapter breaks will split apart the cover page. Then I get this:
 This file is the cover page for the book. This file occurs more than once in the spine.
This file is the cover page for the book. This file occurs more than once in the spine.
I'm usually able to fix this by either re-running the conversion without split or editing the content.opf. 
Is this user error, or a bug?