Translation:She already came yesterday, now she comes again.
You don't really need 'already' in the first part of the sentence.
I think you do need "already" in the first part of the sentence.
It's the translation of "sudah".
If you leave it out, it will be a translation of another sentence.
Like this :
Dia kemarin sudah datang = She already came yesterday
Dia kemarin datang = She came yesterday
Yes, on reflection that must be true, as literal translation, though 'sudah' has a wider range of meanings than 'already'. In UK English I think most of us would leave out 'already' because it would be understood from the 'again' and it makes the sentence sound very clumsy.
I think, if what the speaker really means is that 'she' is being a nuisance, I'd be inclined to say "She only came yesterday but now she's back again today".
She came yesterday or she already came yesterday has the same meaning in English. They say that we need to do this so we know that it is spoken that way in Indonesian, Sudah Datang. But isn't this just promoting poor english? I would like to see what they have in the Duolingo course for Indonesians to study English. I am guessing they use this same material which is wrong. In that case it should be written from the point of view of a native english speaker.
To be honest I don't believe literal translation is valid by itself. Academics like it that way because they believe we all have to use the correct grammar. I am much happier to remain oblivious to the grammar and learn that this english phrase is equivalent to that Indonesian one. Of course we also need to learn the meaning of the words but many of them have multiple meanings in both English and Indonesian so this literal translation they do here often leaves me bewildered as to what they mean. The best would be to have the literal translation alongside a naturally spoken English phrase with the same meaning so we can be sure what the true meaning is.