Okay…I know nothing whatsoever about Duke admissions, so please understand my remarks are speculative and take them with a grain of salt.
Why Duke? I’m curious. You may actually stand out a bit in the applicant pool because as far as I know the only type of debate Duke offers is BP, so it’s not as popular with debaters as other top colleges.
I assume when you say you’ll get a LOR from your coach you mean your debate coach.
Do you have any verification that you speak 4 languages fluently? AP scores? CLEP scores? It IS an impressive accomplishment and having some verification of your claim might help. There are some CLEP exams available this summer. If you have other verification of your abilities that isn’t necessary; I’m just suggesting it if you don’t.
The best thing you can do is get great grades 1st semester. Duke may ask for any first quarter grades if your school gives them. Plus, it’s possible you’ll get deferred and Duke will wait to see your first semester grades before making a final decision on your application. Frankly, I think that due to your Bs in junior year, that’s the most likely outcome for your ED app. That doesn’t mean don’t do it; you’re sending the message Duke is your first choice. But be prepared for being deferred and keep your grades up first semester!
Good luck!