[Python] Django ORM gives duplicates in filtered queryset

I have a django app. I use the ORM to run some queries. It appears I have some duplicates in my result.

While I can simply add a distinct() I would like to understand what is going on.

Here are my models:

class Person(models.Model):
created = models.DateTimeField(auto_now_add=True)
active_stuffs = models.ManyToManyField(Stuff, related_name="persons")
waiting_stuffs = models.ManyToManyField(Stuff, related_name="persons_waiting")
cancelled_stuffs = models.ManyToManyField(Stuff, related_name="persons_cancelled")
# ... other fields

class Stuff(models.Model):
name = models.CharField(null=False, blank=False, max_length=150,)
# ... other fields


Here is the query:

queryset = Person.objects.filter(
Q(active_stuffs__id=some_id)
| Q(cancelled_stuffs__id=some_id)
| Q(waiting_stuffs__id=some_id)
)


What I don't understand, is the following results:

• queryset.count() -> 23
• Person.objects.filter(Q(active_stuffs__id=some_id)).count() -> 16
• Person.objects.filter(Q(cancelled_stuffs__id=some_id)).count() -> 0
• Person.objects.filter(Q(waiting_stuffs__id=some_id)).count() -> 6

An instance of Stuff can only be in either active_stuffs, cancelled_stuffs or waiting_stuffs. I checked the Person instance that is duplicated, and the Stuff instance I'm looking for is only in the waiting_stuffs field... So, where could this duplicate come from?

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