Django Сравните одно поле формы

Question

У меня есть дилемма, у меня есть форма, где необходимо ввести IDENTITY DOCUMENT, но я хочу, чтобы, когда работник компании заполнял это поле и затем отправлял форму, форма выдает сообщение о том, что Клиент существует , потому что IDENTITY DOCUMENT зарегистрирован в системе. Я использую GenericViews.

Форма

класс CustomerForm (forms.ModelForm):

class Meta:

    model = CustomerData

    fields = [

        'name',
        'last_name',
        'dni',
        'phone_number',
        'address',
        'age',
        'birth_date',
        'civil_status',
    ]

    labels = {

        'name': 'Nombre',
        'last_name': 'Apellidos',
        'dni': 'DNI',
        'phone_number': 'Telefono',
        'address': 'Direccion',
        'age': 'Edad',
        'birth_date': 'Fecha de Nacimiento',
        'civil_status': 'Estado Civil',

    }



    widgets = {
        'name': forms.TextInput(attrs={'class' : 'form-control'}),
        'last_name':forms.TextInput(attrs={'class' : 'form-control'}),
        'dni':forms.TextInput(attrs={'class' : 'form-control'}),
        'phone_number':forms.TextInput(attrs={'class' : 'form-control'}),
        'address':forms.Textarea(attrs={'class' : 'form-control'}),
        'age':forms.NumberInput(attrs={'class' : 'form-control'}),
        'birth_date':forms.DateInput(attrs={'class' : 'form-control'}),
        'civil_status':forms.TextInput(attrs={'class' : 'form-control'}),
    }

Модель

Класс CustomerData (models.Model): «»» Модель профиля.

Proxy model that extend the base data
with other informarion.
"""
name            = models.CharField(max_length=50)
last_name       = models.CharField(max_length=50)
dni             = models.CharField(max_length=8)
phone_number    = models.CharField(max_length=20, blank=True)
address         = models.TextField()
age             = models.PositiveIntegerField(default=18)
birth_date      = models.DateField()
civil_status    = models.CharField(max_length=1)

created         = models.DateTimeField(auto_now_add=True)
modified        = models.DateTimeField(auto_now=True)

class Meta:
    verbose_name_plural = "Customer Data"

def __str__(self):
    """ Return username """

    return self.name + " " + self.last_name

View

class RegisterNewCustomer(CreateView):
    model = CustomerData
    form_class = CustomerForm
    template_name = 'customers/customers_form.html'
    success_url = reverse_lazy('customers:customers_list')

class CustomersList(ListView):
    model=CustomerData
    template_name = 'customers/customers_list.html'

class CustomerUpdate(UpdateView):
    model = CustomerData
    form_class = CustomerForm
    template_name = 'customers/customers_form.html'
    success_url = reverse_lazy('customers:customers_list')

class CustomerDelete(DeleteView):
    model = CustomerData
    template_name = 'customers/customers_delete.html'
    success_url = reverse_lazy('customers:customers_list')

Template

{% extends 'base.html' %}
{% block content %}
<form method = "post">
{% csrf_token %}
    {{ form.as_p }}
<button type="submit"> Guardar </button>
</form>
{% endblock %}