Here is a working code

Use the below code snippets in any of your models.py

models.py

from django.contrib.auth.models import User
User._meta.get_field('email')._unique = True

django version : 3.0.2

Reference : Django auth.user with unique email

Working Code for Django 3.1

models.py

from django.contrib.auth.models import User

User._meta.get_field('email')._unique = True
SETTINGS.PY
AUTHENTICATION_BACKENDS = [
     'django.contrib.auth.backends.ModelBackend'
]

There is a great example of this in Django’s docs – https://docs.djangoproject.com/en/2.1/topics/auth/customizing/#a-full-example.

You have to declare the email field in your AbstractBaseUser model as unique=True.

class MyUser(AbstractBaseUser):
    email = models.EmailField(
        verbose_name='email address',
        max_length=255,
        unique=True,
    )
    date_of_birth = models.DateField()
    is_active = models.BooleanField(default=True)
    is_admin = models.BooleanField(default=False)

Easy way:
you can user signal
Example

from django.db.models.signals import post_save, pre_save
from django.dispatch import  receiver
from django.contrib.auth.models import User
from django.forms import ValidationError

@receiver(pre_save, sender=User)
def check_email(sender, instance, **kwargs):
    email = instance.email
    if sender.objects.filter(email=email).exclude(username=instance.username).exists():
        raise ValidationError('Email Already Exists')

You might be interested in:

django-user-unique-email

Reusable User model with required unique email field and mid-project support.

It defines custom User model reusing of the original table (auth_user) if exists. If needed (when added to existing project), it recreates history of applied migrations in the correct order.

I’ll appreciate any feedback.

A better way of doing then using AbstractBaseUser

#forms.py
from django.core.exceptions import ValidationError
from django.contrib.auth.models import User
from django.contrib.auth.form import UserCreationForm
from some_app.validators import validate_email

def validate_email(value):
    if User.objects.filter(email = value).exists():
        raise ValidationError((f"{value} is taken."),params = {'value':value})

class UserRegistrationForm(UserCreationForm):
        email = forms.EmailField(validators = [validate_email])
        
        class Meta:
            model = User
        fields = ['username', 'email', 'password1', 'password2']    
    

In case of use CustomUser model inherit from AbstractBaseUser you can override the full_clean() method to validate unique constraints on the model fields you specified unique=True. This is safer than form (i.e. FormClass) validation.

Example:

from django.contrib.auth.models import AbstractBaseUser
from django.db import models

class CustomUser(AbstractBaseUser):
    email = models.EmailField(unique=True)
    # ...


    def full_clean(self, **kwargs):
        """
        Call clean_fields(), clean(), and validate_unique() on the model.
        Raise a ValidationError for any errors that occur.
        """
        super().full_clean()

Note: Tested on Django 3.1

Improvement for solution with form validation

Instead of raising a ValidationError, it would be better to use the add_error method so that all errors of the forms are sent, and not only the one raised by ValidationError.

class SignUpForm(UserCreationForm):
    email = forms.EmailField(max_length=254, help_text='Required. Inform a valid email address.')

    class Meta:
        model = User
        fields = ('username', 'email', 'password1', 'password2', )

    def clean(self):
        cleaned_data = super().clean()
        email = cleaned_data.get('email')

        if User.objects.filter(email=email).exists():
            msg = 'A user with that email already exists.'
            self.add_error('email', msg)           
    
        return self.cleaned_data

You can edit model in meta as follow

• Note: This will not update the original model

class SignUpForm(UserCreationForm):
    email = forms.EmailField(required=True)

    class Meta:
        model = User
        model._meta.get_field('email')._unique = True
        fields = ("username", "email", "password1", "password2")