It would make sense, since they insist on the Ukrainian transliteration of Kyiv even though Kiev is probably the more common of the two.
That I think is totally legitimate, by analogy with using Gdansk instead of Danzig. But Odesa is less clear-cut; the language you'll most often find spoken on the streets is Russian, in which it's spelt with two s's... so do we transliterate that, or...?
I am not sure why the Duolingo creators are using this spelling; I think they're trying to promote a direct transliteration of the Ukrainian Cyrillic spellings in place names (as noted with 'Kyiv' above). I am a native speaker of English, and all my life I have seen the spellings as "Odessa" and "Kiev."
Odessa should be more correct solution.That`s some sort of mistake.I was in Odessa,and every advert billboard of this city have the two "s",not just one. So, basically it is just a grammatical error.
Please don't start a war about population portions of the city. Odesa is correct UKRAINIAN word.