I think Duncan is underrated, if anything. There is a resistance for many in putting Duncan with the absolute best that ever played, when in reality I think Duncan is right there with them. While it is true that he has been blessed to play in a good system with good talent, it's also true that he has made the most of that opportunity with consistently high impact play. I'd listen to an argument that put Duncan up against anyone that's ever done it.
That said...I still think KG is underrated as well. Because IMO he's even slightly better than Duncan.