def unique_email_validator(form, field):
    user_manager =  current_app.user_manager
    if not user_manager.email_is_available(field.data):
        raise ValidationError(_('This Email is already in use. Please try another one.'))

# ***********
# ** Forms **
# ***********
Username must be unique
dataset/ETHPy150Open lingthio/Flask-User/flask_user/forms.py/unique_email_validator