Validating duplicates in repeating properties for NDB Model

Question

So I have a model based on ndb.Model

from google.appengine.ext import ndb


class CFCSocialUser(ndb.Model):

    def clean_dob(self, value):
        if value.year < 1900:
            raise Exception("Year cannot be less than 1900")

    username = ndb.StringProperty(required=True)
    userid = ndb.IntegerProperty()
    email = ndb.StringProperty(required=True)
    date_of_birth = ndb.DateProperty(validator=clean_dob)

    @staticmethod
    def create_new_user(name, email, date_of_birth):
        app = CFCSocialUser(username=name,
                            email=email,
                            date_of_birth=date_of_birth,
                            id=name)
        return app.put()


    @staticmethod
    def create_new_user_id(name, email, date_of_birth, id):
        app = CFCSocialUser(username=name,
                            email=email,
                            date_of_birth=date_of_birth,
                            key=id)
        return app.put()

I have another model based on ndb.Model, which looks like

from google.appengine.ext import ndb
from mainsite.rainbow.models.CFCSocialUser import CFCSocialUser


class CFCSocialGroup(ndb.Model):
    name = ndb.StringProperty(required=True)
    created_on = ndb.StringProperty()
    created_by = CFCSocialUser()
    members = CFCSocialUser(repeated=True)

How do I add a validator to the members property of CFCSocialGroup, like I validator for the date of birth property in CFCSocialUser using the clean_dob function?

The validator will ensure that duplicate CFCSocialUsers are not part of the same group.