Right! The error message from the grader is pretty clear. Search for the string UNQ_C2 in your notebook using the browser string search (^F or whatever it is in your browser). There must be two occurrences of UNQ_C2. So how did that happen? You must have either duplicated some cell or you purposely or accidentally modified one of the other comment lines that was just given to you.
If you need to get a fresh copy of the notebook to verify how it originally looked, there is a procedure here.