I am sure you have run across a web page that returns nothingness in the past. Yes, a page with no content, no navigation, just a blank white page. This may be the result of a server issue, HTML issue or a security prevention method gone bad.
A Google Webmaster Help has one such webmaster who had this issue and it led to serious Google ranking consequences.
The issue is, I am a bit surprised by Google's JohnMu response. Instead of saying that because Google could not determine the content of the page, they decided to remove it. He said, Google sees multiple blank/empty HTML pages on the site and thus considers them to be duplicate to each other.
It looks like some of the pages from your site have been returning empty HTML pages with almost no content. In general, when this happens, we may assume that you're trying to return this content on purpose, and if it's identical with other content that we've found on your site, we might assume that they're duplicates. If you are not aware of this issue, it may make sense to check with your web-hosters to find out more -- perhaps it's a part of a "security" mechanism?
I would think Google wouldn't consider them duplicate, but rather throw them into a bucket of error pages and recrawl them on a schedule.
Maybe I am misunderstanding. But I felt the difference between an bad error page and a duplicate content issue with a blank HTML page was an interesting difference for SEOs to chew on.
Forum discussion at Google Webmaster Help.