Add distincts to absence query set to prevent double entries

(cherry picked from commit 528e23f0)

CHANGELOG.rst

@@ -15,6 +15,7 @@ Fixed
 * Fix migration names and dependencies
 * Migration path was ambigious
 * Unique constraints for breaks and substitutions were too tight
+* Absences in substitutions list were displayed multiple times.
 
 `2.0b0`_ - 2021-05-21
 ---------------------

aleksis/apps/chronos/managers.py

@@ -600,13 +600,13 @@ class AbsenceQuerySet(DateRangeQuerySetMixin, SchoolTermRelatedQuerySet):
     """QuerySet with custom query methods for absences."""
 
     def absent_teachers(self):
-        return Person.objects.filter(absences__in=self).order_by("short_name")
+        return Person.objects.filter(absences__in=self).distinct().order_by("short_name")
 
     def absent_groups(self):
-        return Group.objects.filter(absences__in=self).order_by("short_name")
+        return Group.objects.filter(absences__in=self).distinct().order_by("short_name")
 
     def absent_rooms(self):
-        return Person.objects.filter(absences__in=self).order_by("short_name")
+        return Person.objects.filter(absences__in=self).distinct().order_by("short_name")
 
 
 class HolidayQuerySet(QuerySet, DateRangeQuerySetMixin):