"If I take the ACT, can I also submit SAT Subject Tests?
Although we do not require or expect SAT Subject Test scores from candidates who take the ACT, we do accept them and will consider them as an additional demonstration of subject proficiency."
Also this:
"If I take both the SAT and ACT, do I need to send both scores to Duke? How does Duke consider multiple scores?
We will always consider your highest score regardless of test type or test date, and you are welcome to send us SAT scores, ACT scores or both. Applicants may choose the Score Choice option for the SAT and choose to submit scores from specific ACT test dates. We do NOT require applicants to submit all of their test scores–just the ones necessary to fulfill the testing requirement."
https://admissions.duke.edu/application/instructions.html#testing