@walkman, did you remove the sitemap in Webmastercentral or added disallow in robots.txt?
I've answers section in my website, for every article we have a question and answer. Question Title may be similar, I believe this caused the problem.
I've few options here:
1. I can use canonical tag, but it is just for the duplicate content. I know that many big sites have comments in a new page with canonical tag. Not sure I can use canonical.
2. Disallow in robots.txt
3. Remove sitemap in WebMasterCentral