Django – duplicate key error after trying to create a new model

I have a location model and it has a city field, which is a foreign key. The database is already filled with all the countries, states and cities so when I want to create a new location I will just select the city, which is connected to a state and the state is connected to a country.

Here is the location model:

class Location(models.Model):
    name = models.CharField(max_length=50, default=None, null=True, blank=True)
    street = models.CharField(max_length=100)
    additional = models.CharField(max_length=100, null=True, blank=True)
    city = models.OneToOneField(City, on_delete=models.CASCADE, related_name="cities")
    zip_code = models.CharField(max_length=30)
    phone = models.CharField(max_length=15, null=True, blank=True)

    user = models.ForeignKey(User, on_delete=models.CASCADE, null=True, blank=True)

    created_at = models.DateTimeField(auto_now_add=True, verbose_name="created at")
    updated_at = models.DateTimeField(auto_now=True, verbose_name="updated at")

    class Meta:
        verbose_name = "location"
        verbose_name_plural = "locations"
        db_table = "locations"
        ordering = ["zip_code"]

    def __str__(self):
        return self.name

    def get_absolute_url(self):
        return self.slug

I am trying to update a location or create it if it does not exists yet and here is the code I use to do that. It’s part of a patch request:

class UpdateUserSettings(generics.UpdateAPIView):
    permission_classes = [permissions.IsAuthenticated]

    serializer_class = SettingsSerializer
    name = 'user-settings'

    def get_queryset(self):
        id = self.kwargs['pk']
        return UserSettings.objects.all().filter(user=id)

    def patch(self, request, *args, **kwargs):
        user = User.objects.get(id=self.request.data.get('user'))
        settings = UserSettings.objects.get(user=user)
        form = self.request.data.get('form')
        if form == 'editEssentialsForm':
            email = self.request.data.get('email')
        elif form == 'editPaymentsForm':
            gateway = Gateway.objects.get(name=request.data.get("gateway"))
            settings.gateway = gateway
            name = "user-" + str(user) + "'s billing address"
            street = request.data.get("billing_street")
            additional = request.data.get("billing_additional")
            city = City.objects.get(id=request.data.get("billing_city"))
            zipcode =  request.data.get("billing_zip")
            Location.objects.update_or_create(name=name, street=street, additional=additional, city=city, zip_code=zipcode, phone=settings.phone, user=user)
            return Response("Settings updated successfully", status=HTTP_200_OK)

When I run the code to create the new location I get the following error:

django.db.utils.IntegrityError: duplicate key value violates unique constraint "locations_city_id_key"
DETAIL:  Key (city_id)=(1) already exists.

There is one other location in the database with the same city so my question is why can I not have two locations with the same city which is what this error is telling me if I’m not wrong?

What am I doing wrong please?