The early decision acceptance rate at Duke is 16.5%, which, while a lot higher than their 5% rate for regular applicants, is still of course very competitive. The only way to guarantee you don’t get in is not to apply of course, but you should definitely approach this as a highly aspirational target to avoid disappointment, if, as is statistically very likely, you don’t get in, and considering that you’ll be competing against lots of applicants with near-perfect grades and a long list of extracurriculars. The US admissions process is just inherently a lot more unpredictable, opaque and, frankly imo, unfair than the UK one. The other thing to bear in mind about early decision is that its major disadvantage is that the college is much less likely to be generous with financial aid if they do accept you, as you are not then allowed to apply to any other US college that year who might give you more financially enticing offers.
On the SAT and ACT, Duke requires you to take one of them. However, fortunately it appears it may not be too late for you, as the deadline has not yet passed to take one of them before you apply. Finally, a bit of a nitpick, but Duke is not Ivy League.