We are facing the same problem with our parenting website in Germany. We go all the K2-features working and looking as we wished them to. As I read about your experience, we stopped all our export plans because we cannot risk our SEO standing with this project.
Today I stumbled into this extension:
extensions.joomla.org/extensions/extension-specific/xmap-extensions/10297
Would that help us arranging a K2 Sitemap and avoiding thus Google to see the content only once, as Giu Tae Kim suggested????

Any idea or experience about that.
Is there a moderator out there???