This should be "khỏe", right? I've seen a couple of "khoẻ"s so far, and I'm reporting them, but I want to make sure it is, in fact, a typo.
For these words, there are two ways to put the tone marks: old and new ways. This course uses the new way, that is the tone marks are put on/under the second vowel in oe, oa and uy (e.g: khoẻ, thuỷ, doạ, etc.). If you use the old way, Duo will show it as a typo.
I see! It's confusing, because my computer changes "khoẻ" to "khỏe" automatically, but my iPhone auto-corrects "khỏe" to "khoẻ" (both use TELEX) . :-P Since there are a lot more google results for "khỏe" than "khoẻ" (and "khoẻ" isn't listed as a word at VDict.com), I assumed it was incorrect. Thanks for the explanation!
Alas, I'm using linux (ubuntu), and it does not appear that this version of Unikey has an option for "modern orthography". I think I will have to just survive with typo messages for now.