"This shirt is more expensive than that shirt."
Given that the English says "shirt" twice, I feel that while either translation version is correct for meaning, it is more accurate to put 衬衫 twice in the Chinese version -- and not once. In any event, either version should be correct, and Duolingo should amend their automatic answer here.
I'm not sure about what I'm going to say and I hope a native will soon come to say if I'm right or wrong.
While it is okay in English to say both "...that one" or "...that shirt" in this kind of situation, I feel like it would be very unnatural in Chinese to repeat 衬衫 a second time. I don't recall anybody speaking this way, and it goes against Chinese rule n°1: Simple and short!!!
I don't know if this thread connects to another module, but mine is translating English to Chinese, so to me you're asking to change the given English to better match the Chinese translation. But I think this helps us see the difference in the two languages; i.e. it's more common to have repeated items (shirt here) in English than in Chinese. (You'll forgive me if I'm mistaken here :)