One has to go back at least 40 years to find a meaningful difference in academics and selectivity, I suspect. Duke has been a top 10 school in the USNWR rankings every year since 1985 with only one exception (#12 in 1989).
Duke was not as much of an academic powerhouse in the 1960s, though it was already pretty strong. In the 1964 Cartter Report, Duke ranked #20 for departmental strength, coming in 11th among private universities (behind Harvard, Yale, Princeton, Stanford, Chicago, Columbia, Cornell, JHU, Penn, and Northwestern, in that order).