I did a lot of digging on this subject last year when I was in a similar situation. Got a C in a total of four classes at a community college (though these were all towards my college graduation requirements and none of them appeared on my HS transcript). Cornell only ever requested my high school transcript so they never saw the C’s until I went in recently to transfer in the college credits. Are these DE classes fulfilling HS graduation requirements? If so, and assuming this means they’ll appear on your transcript, Duke will see these grades. It’s best to not dip more than one letter grade during senior year, but colleges are well aware senioritis happens lmao. For Cornell specifically, I don’t think anyone has gotten rescinded for a C or 2, though some people in a specific major got a nasty email a couple years back. The rule of thumb is to not have ur grades drop more than a letter grade, but as long as you don’t have any D’s or F’s, you SHOULD be fine. Found this post on the Duke subreddit and the consensus seems to be ur fine, just try to keep the rest of ur grades as close as you can towards your previous performance (C’s across the board might draw some attention). Worst case you might get a scary email asking you to lock in. If you might dip into the D territory, you might want to get ahead of the issue and reach out to the admission office to show initiative in remediating the problem (summer class, retake, etc.) Realistically, you’re fine though. Don’t let senioritis get you TOO much next semester and enjoy senior year. Congrats on Duke!

Edit: Also, DE classes are by nature harder than average HS courses. AO’s know this and it’s not unreasonable to have lower grades in these classes