def METHOD_NAME(self):
    User.objects.create_user('test1', 'test@example.com', 'testpass')
    form = SignupForm({
        'username': 'username2',
        'email': 'test@example.com',
        'password1': 'password',
        'password2': 'password'
    })
    self.assertFalse(form.is_valid())
    self.assertIn('email', form.errors)