Translation:What is the tall boy doing in front of that beautiful Brazilian girl?
I wrote, "What is the tall boy in front of that beautiful Brazilian girl doing" and it was rejected, in favor of the above. They mean different things. In my translation, it asks "what is the boy doing?" He is identified as the one who's in front of the beautiful Brazilian girl, but that is the only reason she is mentioned. In the Duolingo translation, it asks what he's doing in front of her, implying that being in front of her is at least part of the reason for his actions.
Could the Hungarian sentence be interpreted either way?
(I'm assuming it does NOT mean, "How did he get in front of her? He's not supposed to be there," etc.)
I don't understand this question. "Az" has nothing to do with the vowels in előtt. It's part of a construction: Az előtt a szép brazil lány előtt. Előtt is there twice, once after the Az, and once after the noun (including the adjectives in front of the noun).
It's true that Duo has been inconsistent about whether to translate this construction to "the" or to "that," especially in the sentences with relative pronouns. In English those sentences work better with "the" because the second part of the sentence after the relative pronoun, does the job of identifying the subject, so "that" is redundant. But in this case, the word "that" is appropriate in English, too.
Are you using the mobile app of Duolingo? Many of these exercises have a "Tips and Notes" section, but they're not available via the app, for some reason. If you can log onto your Duolingo account via a web browser, you'll be able to see it. I remember learning about the "az a [postposition] a [noun] [postposition]" construction in the Tips and Notes for that section.
I don't have an explanation for why it has to appear twice in this construction. I'm sure there's a history to it, but I don't know what that is. (Someone who knows more about that might be able to say more.) That's just how it is. It's not a mistake.