This is not entirely true. Let me explain...
Duplicate entries depend on your menu structure. If you have a menu that maps all your K2 categories (or at least the most imporrtant ones), then you won't have duplicate entries.
To further mitigate that, one of the things we want to add in K2 v2.5.7 is canonical URLs to prevent Google from seeing duplicate item entries (if they exist).
That should further improve K2's SEO features.
Unlike the default Joomla! content or other CCK extensions, K2 already sports much better SEO. From the "read more" links which contain the item's title, to the images, to author links etc. etc. ;)