While it is somewhat strange to hear two "readys" in a row, I believe that most comments on this lesson are missing the intention of già - that lunch came sooner than expected, not that lunch is ready "now". I think it important to recognize the Italian speech patterns here rather than the English. However, in English this phrase might be used more as a question indicating surprise than a statement as Duo created it.
Probably me. Just like I have known myself to say "...ready already?"
We all agree that the phrase sounds strange and that is a lesson for us all - be more specific with our words.
The major problem I see is that "done" is has many meanings and is too general in its meaning. When you say "done" do you mean ended (completed, executed, discharged or finished)? Or do you mean cooked (prepared, fixed, produced and served )? Is it being used as a past tense verb or an adjective?
"Ready" has the same problem. Does it mean prepared for action or available for immediate use? Is it disposed or inclined to do something or is it on hand?
I urge both Duo and the students to choose words wisely. Use any of the synonyms to to better communicate intent.