Add distinct to group-based statistic views (extra marks and excuse types)

This advances #219 (closed). I forgot to add distinct at two places.

aleksis/apps/alsijil/model_extensions.py

@@ -471,6 +471,7 @@ def generate_person_list_with_class_register_statistics(
                 extra_mark.count_label: Count(
                     "filtered_personal_notes",
                     filter=Q(filtered_personal_notes__extra_marks=extra_mark),
+                    distinct=True,
                 )
             }
         )
@@ -484,6 +485,7 @@ def generate_person_list_with_class_register_statistics(
                         filtered_personal_notes__absent=True,
                         filtered_personal_notes__excuse_type=excuse_type,
                     ),
+                    distinct=True,
                 )
             }
         )