http://gravitytales.com/novel/a-dragons-curiosity does the same thing. In both cases, there's a section called 'Chapters' with chapters, then named sub-book sections that again contain the same chapters--in the case of a-dragons-curiosity, the sub-books
don't contain all the chapters in 'Chapters'.
OTOH, the other 5 on the drop down at the top of the page under 'Original Tales' and a handful spot checked from 'Translated Tales' don't have a 'Chapters' section and don't appear to have any duplicates.
So my assumption is that authors did that on purpose for some reason, rather than it being a bug as such.
If it's a common thing, I suppose code to prevent duplicate chapters could be added to the adapter. I don't remember it being an issue before.