This is what I found over Duke site -

“Enroll in the best available and most challenging courses.We recommend four years of English and at least three years of mathematics, natural sciences, foreign language, and social studies. Application Overview - Duke Admissions"

从字面上看，至少要三年。

所有跟帖： 

• 同意老杨的，是指3年的level，我们AP考过就不再上外语课了，已经最高level没课课上了 -dengGGdeng- ♀ (0 bytes) () 09/04/2018 postreply 23:20:54

• 我儿子中文一二考的。三上的课。没其他外语。录取duke没问题。没觉得大学会看这个。外语只是个选项 -Dancingpanda- ♀ (0 bytes) () 09/05/2018 postreply 14:07:21