Hi Igor,
I'm not sure what could cause that. We are basically concatenating the item title and link and using a hashcode of the resulting string as a way to identify if an item already exists or not. It looks like the title and link are identical so it should produce the same hash code and therefore not create a duplicate in the mp_RssFeedEntries table. I would look at the rows there and see if they have different or the same hash code.
Hope it helps,
Joe