Just a few questions ago, both "Sei l'unico" and "Sei l'unica" were given as correct translations in the answer box for the English sentence "You are the only one". I don't see why they wouldn't both work here as well. I got the version of this question in which we have to choose either " l'unico" or " l'unica" to fill in the blank: Non sei ___. They both seem right to me. How do you choose only one?
you're mistaken. 'l'unico' is masculine and 'l'unica' is feminine not 'la unica'. here is a website that may help. http://www.uvm.edu/~cmazzoni/3grammatica/grammatica/articles.html
I believe it's because it is l'unico and not just unico. So it would read as "you are not -the- unique" in English which doesn't make sense, the "one" is implied from the "the". Next time it comes up I'm going to try "you are not the unique one" and see if it is accepted.