The UCs have a penchant for standardization in various admissions procedures, and
this article was written over 12 years ago, but I can definitely attest to their assessment of Duke's toughness (they seem to have ranked it as the third most difficult, behind only Swarthmore and Williams). From my experiences in a difficult humanities Focus cluster to my friends' trials in engineering, it's tough to pull consistent A's here. Obviously, I can't knowledgeably affirm all the other school's "rankings," but Duke'll be tough on your GPA. Be willing to suck it up.