Grade school exercise: A student cannot simultaneously be in two grades since the users' names are unique (exercism#1735)

Author: pedrorolo

exercises/grade-school/canonical-data.json

@@ -14,6 +14,19 @@
             },
             "expected": ["Aimee"]
         },
+        {
+          "uuid": "dece43c8-3ba5-11eb-8fdf-7f8daeaeb5f2",
+          "description": "A student can't be in two different grades",
+          "property": "roster",
+          "input": {
+            "students": [
+              ["Aimee", 2],
+              ["Aimee", 1]
+            ],
+            "desiredGrade": 2
+          },
+          "expected": []
+        },
         {
             "uuid": "233be705-dd58-4968-889d-fb3c7954c9cc",
             "description": "Adding more students adds them to the sorted roster",