Translation:My uncle and my aunt will be your friends.
You must report it. Languages like Italian and German too e.g., in which a noun's grammatical gender dictates adjective and article endings require repetition of these parts of speech; English as you know does not and so omitting the possessive adjective, as unnecessary, is not only perfectly acceptable, but is in fact standard. It's its inclusion in DL's translation that sounds unnatural and awkward.
Report it again -- the complaint, not the possessive! :-). The only way you'd need to repeat the possessive adjective in English is if the two people aren't a pair: My aunt from Boston and my uncle from New York both came to the wedding. But if they're a couple, English would not repeat the possessive: My aunt and uncle from Chicago both came to the wedding. As I said above, Italian like German where nouns have gender, require use of the possessive with both nouns, why? "aunt and uncle" isn't some sort of hyphenated noun with a single gender.
Duolingo may not be speaking of a married aunt and uncle. Perhaps an uncle from one side qnd aunt from the other side of the family. In this case you would indeed say "my uncle and my aunt" so as not to give the incorrect assumption that th are a pair. This may be why you are being marked incorrect.