Resolve "Show abbreviations in the substitution plan only once"

Closes #140 (closed)

aleksis/apps/chronos/managers.py

@@ -545,7 +545,7 @@ class LessonSubstitutionQuerySet(LessonDataQuerySet):
         """Return all lessons which are affected by selected substitutions."""
         from .models import Lesson  # noaq
 
-        return Lesson.objects.filter(lesson_periods__substitutions__in=self)
+        return Lesson.objects.filter(lesson_periods__substitutions__in=self).distinct()
 
     def affected_teachers(self):
         """Get affected teachers.
@@ -553,13 +553,21 @@ class LessonSubstitutionQuerySet(LessonDataQuerySet):
         Return all teachers which are affected by
         selected substitutions (as substituted or substituting).
         """
-        return Person.objects.filter(
-            Q(lessons_as_teacher__in=self.affected_lessons()) | Q(lesson_substitutions__in=self)
-        ).order_by("short_name")
+        return (
+            Person.objects.filter(
+                Q(lessons_as_teacher__in=self.affected_lessons()) | Q(lesson_substitutions__in=self)
+            )
+            .distinct()
+            .order_by("short_name")
+        )
 
     def affected_groups(self):
         """Return all groups which are affected by selected substitutions."""
-        return Group.objects.filter(lessons__in=self.affected_lessons()).order_by("short_name")
+        return (
+            Group.objects.filter(lessons__in=self.affected_lessons())
+            .distinct()
+            .order_by("short_name")
+        )
 
 
 class DateRangeQuerySetMixin: