The word schon is called a particle and is often used to add 'flavour' to a sentence (it has some other functions too). It's hard to get an idea for its usage without being exposed to conversation in German. Literally, it means "already". But here as a phrase schon wieder is something slightly exasperated like "AGAIN?!".
In that case it's not clear if the English translation given is good. If I cook today and am supposed to cook again next month, then it's fair to ask if I'm supposed to cook again. But if I just finished, and I indicate that I will start cooking in a minute, someone might ask if I'm already supposed to cook again.
Is "are you supposed to cook again" really a good translation? It seems as if "Are you supposed to cook yet again" is closer in meaning.
Duo's translation does nothing to capture the essence of "schon", then, and should imho be amended to convey the effect of "schon" here.
Additionally if Duo wants to teach us these "flavor particles" it would be really quite helpful to do so in a specific unit, as currently you only encounter them once in a blue moon, which makes learning/memorizing them unnecessarily difficult, especially since you also see them used in their more "normal" (?) sense, making it rather difficult to identify exactly when they're there only to 'flavor' the sentence.
(Best that I can tell is they're sometimes used 'normally', sometimes for 'flavor', and I have no idea how I am supposed to tell when they're being used which way.)
Question about 'Sollen' - in a lot of sentences Duolingo translates it as 'supposed to' rather than 'should'. I feel like there's a big difference in meaning between these; 'supposed to' suggests an outside force making you do it or adhering to rules, whereas 'should' implies that you may be doing it out of your own desire.
"Are you supposed to cook again? The boss was really unclear!"
"Should you cook again? It was really nice last time!"
Does 'Sollen' work in both these ways and determined by context? Or does it have a tendency towards meaning one or the other?
Looking only at the grammar, "should" corresponds to "solltest" in German, while "sollst" would be literally "shall". If you ask "Should you [..]?", to me it sounds like asking someone if he thinks it would be a good idea that he cooks again, while the German sentence asks whether someone else wants him/her to cook again. Am I wrong?