Evaluating Tim Duncan: the KG problem

Tim Duncan is a consensus top-10 player, with some people putting him in the top-5. The resume speaks for itself. The problem is that he had a contemporary peer; Kevin Garnett.

Most top 10 players do not have a contemporary who could replace them on most of their rosters and experience similar success. If you expand that out to the top 20 to 30, most of the guys who would have similar careers if their situations were swapped are usually rated right next to each other (Barkley/Malone). The one exception seems to be Russell/Chamberlain, and that is a very contentious argument. However Duncan and Garnett are not rated near each other, and the argument is not particularly contentious. I think most people rate Kevin Garnett as a top 25 guy.

So my question is, are we underrating Kevin Garnett or overrating Tim Duncan?