Translation:I do not want a shirt because I want a skirt.
Funny linguistic point...not about DUO, but about Portuguese: Why do you need Eu...the sentence would mean the same, and be totally understandable without them....a literal translation of the sentence above is:
I don't I want a shirt, I don't I want a skirt.
Funny way of doing things.
Actually, the literal translation is exactly the one Duolingo suggested: "I don't want a shirt because I want a skirt". I don't know where did you get those 4 Is from. The conjugation implies the person, but you don't translate, literally, the conjugated verb as a subject + verb in English just because of that. But you can omit the Eu in the original sentence, both of them, although if you omit the first it may sound weird for native speakers.