Hi Roberto,
Before you buy a third party SEF extension like sh404sef which still causes duplicate errors, am I correct in guessing the URL your talking about are coming from category links ?
If so try making a hidden menu and add the actual K2 Item to this menu, the menu doesn't need to be published to your site, I get URL's without ID & Item as long as I do this and have the right setting in Global Configuration
Search Engine Friendly URLs: Yes
Use URL rewriting Yes. (Rename htacess.txt if needed)
let me know how you go Pete.