IntegrityError column email is not unique

I am working on my Checkout view with regular/guest user but getting hard time to come around the integrity error. Idea is to let guest users register with email only to checkout and I need to set the user email unique. 

models.py

from django.conf import settings  from django.db import models    class UserCheckout(models.Model):      user = models.OneToOneField(settings.AUTH_USER_MODEL, null=True, blank=True)      email = models.EmailField(unique=True)        def __unicode__(self):          return self.email

forms.py



from django import forms  from django.contrib.auth import get_user_model    User=get_user_model()  class GuestCheckoutForm(forms.Form):      email = forms.EmailField()      email2 = forms.EmailField(label='Verify Email')      def clean_email2(self):          email = self.cleaned_data.get("email")          email2 = self.cleaned_data.get("email2")          if email == email2:              user_exists = User.objects.filter(email=email).count()              if user_exists != 0:                  raise forms.ValidationError("User already exists. Please login instead")              return email2          else:              raise forms.ValidationError("Please confirm emails addresses are the same.")

In my cart views this is how I've rendered my form.

def post(self, request, *args, **kwargs):          self.object = self.get_object()          form = self.get_form()          if form.is_valid():              email = form.cleaned_data.get("email")              user_checkout = UserCheckout.objects.create(email=email)              return self.form_valid(form)          else:              return self.form_invalid(form)

I've registered the model with admin and in admin it shows the error for duplication perfectly fine but from frontend I am getting error below:

IntegrityError at /checkout/  column email is not unique  Request Method: POST  Request URL:    http://localhost:8000/checkout/  Django Version: 1.8.13  Exception Type: IntegrityError  Exception Value:      column email is not unique  Exception Location: C:\Users\Ali\ecomm\lib\site-packages\django\db\backends\sqlite3\base.py in execute, line 318  Python Executable:  C:\Users\Ali\ecomm\Scripts\python.EXE  Python Version: 2.7.9

Please advise.

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