Django - A user with that name already exists

I am currently learning Django framework and im doing a little project for learning purposes. I have a model class named "Conta" and it has a foreign key to the User model (the default django User). What happens is this: Whenever i try to add a new row to the model "Conta", it throws an error saying that the user already exists, even though i am not trying to create a new user. I will post my code:

SERIALIZER for the model Conta and User:

class ContaDetailsSerializer (serializers.ModelSerializer):
    """Lists the details of a conta"""
    tipo_moeda = TipoMoedaSerializer()
    tipo_conta = TipoDeContaSerializer()
    user = UserSerializer()

   # user = UserSerializer()
    class Meta:
        model = Conta
        fields = ['id','user','nome','balanco','numero','tipo_moeda','tipo_conta','notas']

    def create(self, validated_data):
        tipoConta_data = validated_data.pop('tipo_conta')
        tipoMoeda_data = validated_data.pop('tipo_moeda')
        user_data = validated_data.pop('user')
        print(tipoMoeda_data['id'])
        tipo_conta = get_object_or_404(TipoConta,pk=tipoConta_data['id'])
        tipo_moeda = get_object_or_404(TipoMoeda,pk=tipoMoeda_data['id'])
        user = get_object_or_404(User,pk=user_data['id'])
        conta = Conta.objects.create(**validated_data,tipo_conta=tipo_conta,tipo_moeda=tipo_moeda,user=user)
        return conta


class UserSerializer(serializers.ModelSerializer):
    """Lists a user"""
    id = serializers.IntegerField()
    class Meta:
        model = User
        fields = ['id','username

']

View for the creating of a new "Conta":

class ContaList(APIView):
    def post(self,request):
        """Creates a Conta"""
        serializer = ContaDetailsSerializer(data=request.data)
        if serializer.is_valid():
            serializer.save()
            return Response(serializer.data,status=status.HTTP_201_CREATED)
        return Response(serializer.errors,status=status.HTTP_400_BAD_REQUEST)

This is using the Django REST Framework.

What am i doing wrong ? The TipoConta and TipoMoeda are foreign keys too, and they work perfectly.