I just had a brainfart and wrote "them" instead of "you" and it incorrectly accepted it as correct. Hrrmph ... there's really no way to report an incorrect answer being accepted.
(I had another brain-fart and somehow wrote "two" instead of "wrote" ... ?? Ninahitaji usingizi zaidi!)
There are three ways of clarifying this wa (you/them) infix ambiguity:
Anawasaidieni as in the given sentence.
Anawasaidia ninyi specifying the personal pronoun.
Anakusaidieni, using the singular 'you' infix, but the plural imperative construction.
The first method is by far the most common.
Yeah, it's essentially a little thing you put on the end of a verb (or some greeting phrases) just to indicate that you're talking to more than one person, so it only appears on verbs where it would otherwise be ambiguous.
Imperatives would otherwise be ambiguous as to plural or singular because they don't have a subject prefix.
When the object is just indicated with -wa-, that's also ambiguous, so it can be optionally used then to make it clear that it's "you (pl)" and not "them".
I guess that till now nowbody knows - me also not. I think it isn't the form which differents " you" and "you all". But I suppose that it signes plural form. The ending - ni ( - eni ) is the most similar to the form of imperative plural. So, as usually Duolingo gives somethng without giving any explanation - ninaipenda !