gistfile1.py

class Course(models.Model):
    name = models.CharField(max_length=255)


class Teacher(models.Model):
    name = models.CharField(max_length=255)
    courses = models.ManyToManyField(Course)


class M2MDjangoFilterBackendTests(TestCase):
    def test_duplicate_results(self):
        class CourseSerializer(serializers.ModelSerializer):
            teacher_set = serializers.PrimaryKeyRelatedField(many=True)

            class Meta:
                model = Course

        class TeacherSerializer(serializers.ModelSerializer):
            class Meta:
                model = Teacher

        class TeacherView(generics.ListCreateAPIView):
            serializer_class = TeacherSerializer
            model = Teacher
            filter_backends = (filters.DjangoFilterBackend,)
            filter_fields = ('courses',)

        class CourseView(generics.ListCreateAPIView):
            serializer_class = CourseSerializer
            model = Course

        view = CourseView.as_view()
        request = factory.post('/', {'name': 'Course 1'})
        course1_response = view(request)

        view = CourseView.as_view()
        request = factory.post('/', {'name': 'Course 2'})
        course2_response = view(request)

        view = TeacherView.as_view()
        request = factory.post('/', {
            'name': 'Teacher 1',
            'courses': [
                course1_response.data['id'],
                course2_response.data['id']
            ]
        })
        teacher_response = view(request)

        view = TeacherView.as_view()
        request = factory.get('/', {'courses': 1})
        teachers_response = view(request)

        self.assertEqual(teachers_response.data, [teacher_response.data])
        self.assertEqual(len(teachers_response.data), 1)