# -*- coding: utf-8 -*-
from __future__ import absolute_import, unicode_literals

from django import forms
from django.contrib import admin
from django.contrib.auth.admin import UserAdmin as AuthUserAdmin
from django.contrib.auth.forms import UserChangeForm, UserCreationForm

from .models import User


class MyUserChangeForm(UserChangeForm):
    class Meta(UserChangeForm.Meta):
        model = User


class MyUserCreationForm(UserCreationForm):
    class Meta(UserCreationForm.Meta):
        model = User

    def clean_username(self):
        username = self.cleaned_data["username"]
        try:
            User.objects.get(username=username)
        except User.DoesNotExist:
            return username
        raise forms.ValidationError(self.error_messages['duplicate_username'])


class UserAdmin(AuthUserAdmin):
    form = MyUserChangeForm
    add_form = MyUserCreationForm


admin.site.register(User, UserAdmin)

# -*- coding: utf-8 -*-
from __future__ import absolute_import, unicode_literals

from django import forms
from django.contrib import admin
from django.contrib.auth.admin import UserAdmin as AuthUserAdmin
from django.contrib.auth.forms import UserChangeForm, UserCreationForm

from .models import User


class MyUserChangeForm(UserChangeForm):
    class Meta(UserChangeForm.Meta):
        model = User


class MyUserCreationForm(UserCreationForm):
    class Meta(UserCreationForm.Meta):
        model = User

    def clean_username(self):
        username = self.cleaned_data["username"]
        try:
            User.objects.get(username=username)
        except User.DoesNotExist:
            return username
        raise forms.ValidationError(self.error_messages['duplicate_username'])


@admin.register(User)
class UserAdmin(AuthUserAdmin):
    form = MyUserChangeForm
    add_form = MyUserCreationForm

Make use of 'admin.register' decorator in admin.py