Nested subquery distinction to annotate sum

Question

So I'd like to annotate a sum to a queryset, which has a little calculation rule with vars from remote tables.

    facturen = Factuur.objects.values('inkoopopdracht').filter(
        inkoopopdracht=OuterRef('pk'),)
    gefactureerd = facturen.annotate(
        gefactureerd=Sum(Case(
            When(
                factuurpost__inkooppost__btw__systematiek=2,
                then=(F(
                    'factuurpost__inkooppost__aantal')*F('factuurpost__inkooppost__eenheidprijs'))),
            default=F(
                'factuurpost__inkooppost__aantal')*F('factuurpost__inkooppost__eenheidprijs')*(
                    1+F('factuurpost__inkooppost__btw__percentage')),
            output_field=DecimalField(),
            )),
        ).values('gefactureerd')
    qs = qs.annotate(
        factuursom=Subquery(gefactureerd.values(
            'gefactureerd'), output_field=DecimalField()),
        )

The result of the above query is satisfactory. Except when there is a multiple of 'factuurpost'. In this case the sum seems to be multiplied with factuurpost instances.

A fix for this, would be 'distinct=True' on the Sum. However since it only distincts values and not instances, this introduces a new problem. Similar values are no ignored, yielding the wrong sum.

This seems to be an issues other people have come across with as well, e.g. : Django Count and Sum annotations interfere with each other

Now, I used the solution there, with the subquery. I tried to nest the subquery to no avail. Does anyone see a potential solution to have it calculate the right sum in all cases? Thank you in advance!

EDIT The models:

class Factuur(models.Model):

    inkoopopdracht = models.ForeignKey(InkoopOpdracht, models.CASCADE)
    ...


class FactuurPost(models.Model):

    factuur = models.ForeignKey(Factuur, models.CASCADE)
    inkooppost = models.ForeignKey(InkoopPost, on_delete=models.CASCADE)
    aantal = models.IntegerField()
    ...

class InkoopPost(models.Model):

    inkoopopdracht = models.ForeignKey(InkoopOpdracht, models.CASCADE)
    aantal = models.IntegerField()
    eenheidprijs = models.DecimalField(max_digits=10, decimal_places=2)
    ...