Proposed Grades - Prevent students from proposing a grade that already exists

Students that are used to using the Propose Grade option as a way to provide evidence are proposing grades that already exist (ie. Student has a score of 3 and wants to upload additional evidence so they propose another grade of 3 and add another file upload.

This has resulted in some schools having 1000+ proposed grades to approve for no reason. Instead we should prevent students from doing this.