This seems to be the classical situation where Duo errs and ends up creating a complete chaos among users because a definite answer cannot be found --for obvious reasons.
In the following three threads, Duo said that "I want him to work on everything" should be translated as "Voglio che ci lavori lui su tutto," or viceversa.
- Voglio che ci lavori lui su tutto
- Voglio che ci lavori lui su tutto
- I want him to work on everything
While many answers were given to try to explain the presence of both the particle "ci" and the pronoun "tutto," none really explains it, except the one saying it is a mistake, which I agree with and will try to prove that is so.
The particle "ci" has quite a number of uses, and one of them is as an indirect complement, which can also have different meanings depending on the context:
- in quel luogo
- con lui/lei/loro
- in/a/su questo
So, "ci lavoro da 3 mesi," may mean:
- I've been working there (in quel luogo) for three months
- I've been working with him (con lui) for three months
- I've been working on it (su quello) for three months
Just as a side note, the use of "ci" as a complement to specify the receiver of an action (complemento di termine), is not acceptable, for example: *ci dico (dico a lei/lui/loro).
Before arriving to the sentence in question, I'll list six sets of examples to show some valid combinations.
- lavoro + a few prepositions
- so che lavori per lui - I know that you work for him
- so che lavori con lui - I know that you work with him
- lavoro a un progetto personale - I work on a private project
- vuoi che lavori nel fine settimana? - do you want me to work on the weekend?
- lavoro + su
- lui lavora su una stazione spaziale - he works on a space station
- voglio che lavori su questa traccia - I want you to work on this track
- lascia che lavori su di lei - let me work on her image
- voglio che lavori su quell'articolo - I want you to work on that article
- lavoro + sopra
- una celebrazione della gente che lavora sopra e sotto terra - a celebration of the people that work on, and under, the land
- lui intraprende lavori sopra e sotto coperta - he undertakes jobs above and below deck
- ci lavoro + a few prepositions
- meglio che ci lavori anch'io - I better work on it, too
- diciamo che ci lavori con impegno - we say you work hard on it
- lui vuole che ci lavori subito - he wants you to work on it immediately
- devo vendere la masseria e mi serve qualcuno che ci lavori qualche giorno - I have to sell the finca and I need someone to work there for a few days
- ci lavoro + su
- è meglio che ci lavori su - you better get on it
- quando avrai una pausa durante il caso, voglio che ci lavori su - when you get a break in the cause, I want you working on it
- ci lavoro + sopra
- ci serve che ci lavori sopra - we need you to stay on this
- non voglio che ci lavori sopra tutta la notte - I don't want you working all night on it
- se ci lavoro sopra le prossime due pause, penso che potrei finirlo - if I work on it the next two breaks, I think I might finish it
- ed essere un autore con un accordo di pubblicazione non vuol dire proprio voler far arrivare le proprie canzoni a qualcun altro che ci lavori sopra? - and isn't the whole point of being a songwriter with a publishing deal that you can get your songs out there for someone else to work on?
As shown, there are examples available with "ci + lavorare + su/sopra," but they are definitely less than those without "ci." Compare the following Google search results.
Without "ci:"
With "ci:"
ci lavoro su: 10,800 results (0.13% of the cases without "ci")
ci lavori su: 3,490 results (0.20% of the cases without "ci")
ci lavora su: 10,100 results (0.97% of the cases without "ci")
From this, I'd say that eventhough the "ci + lavorare + su/sopra" combinations are used, maybe they aren't really that valid, at least from a strict grammatical point of view, which to me would seem more than reasonable because the "ci" is already indicating the "on it" part, making the "su" or "sopra" redundant (or viceversa, the latter making the former innecessary).
Ok, now back with Duolingo, let's notice that in a couple of instances it used the correct translation, without any kind of redundancy:
Non voglio che lei lavori su tutto - I don't want her to work on everything
Voglio che ci lavori lui - Quiero que trabaje él en eso (I want him to work on it)
Let's see now what Google says about ci lavori lui su tutto: 6 results! As seen, most come from Duo's forum, and the rest are from either questions made elsewhere about the sentence, or from flashcards created with it.
I must say that I'm not completely convinced about it being correct (as an explanation for Duo's sentence), but the answer given on StackExchange is the only one I think I would consider to at least have some possibility of being an acceptable answer:
In this sentence, particle "ci" has the meaning of "su tutto". It is used in a pleonastic way because the sentence of your question contains a "dislocazione a destra", a construction typical of oral speech which is explained in detail in this answer.
As in this example given in that question: "Non ci sono andato, a Venezia."
which comes from the book Grammatica dell'italiano adulto by Vittorio Coletti, the sentence has a complement ("a Venezia" in Coletti's example, "su tutto" in your sentence) that goes after a complete clause ("Non ci sono andato" in Coletti's example, "Voglio che ci lavori" in your sentence) and which is anticipated in this clause by particle "ci".
Of course, for this to work I think the comma is needed: voglio che ci lavori lui, su tutto.
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