I'm a current Duke student and would be glad to try and answer this question. The last average Step 1 score I've heard was 234 (national average of ~217), from 1-2 years ago. This supposedly placed Duke at the second highest in the nation, after Penn, in Step 1 scores. We've been told that this year the average is closer to 240, although I don't know the specific number. But the basic message is that clear: Duke students do VERY well on Step 1! In part it may be due to the fact that we're not required to take the boards at a specific time in medical school, and thus can tailor our studying as we wish. We also get to take Step 1 AFTER we've already done our core clinical clerkships, which I believe provides an advantage on the boards. (although Step 1 is billed as a "basic science exam," its content is very clinical, so early exposure makes a difference)
I hope that helps 🙂