Change CheckCellMetadata error message to mention possible duplicate ids

Author: tuncbkose

nbgrader/preprocessors/checkcellmetadata.py

@@ -13,7 +13,8 @@ def preprocess(self, nb: NotebookNode, resources: Dict) -> Tuple[NotebookNode, D
             MetadataValidator().validate_nb(nb)
         except ValidationError:
             self.log.error(traceback.format_exc())
-            msg = "Notebook failed to validate; the nbgrader metadata may be corrupted."
+            msg = "Notebook failed to validate; the nbgrader metadata may be corrupted " \
+                  "or a cell might have been duplicated."
             self.log.error(msg)
             raise ValidationError(msg)