I would welcome an Ecclesiastical Latin course here too! I am interested in the differences!
At this basic level the only real difference would be pronunciation, which you can easily find demonstrations of on YouTube, and if you learn how to pronounce one the other is easy to understand or learn to pronounce if you want to.
There are a few very minor differences in spelling conventions.
Ecclesiastical Latin at a fairly advanced level has a somewhat different vocabulary than classical Latin, of course, as it is concerned with different subjects and concepts and as it has been written for almost 2000 years, whereas classical Latin is from only a century or two.
In re grammar, ecclesiastical Latin is wannabe classical Latin that isn't quite as complex.
If you learn classical Latin you will not have trouble with ecclesiastical Latin, especially at any level Duolingo might ever reach.
Duolingo's course is just fine as an introduction to ecclesiastical Latin, excepting the few differences in pronunciation between the two. Don't be concerned. Just enjoy learning beginning Latin here.