'Read and return the next record from C{self._users}, filtering out
any records with previously seen uid values (as these cannot be
found with C{getpwuid} and only cause trouble).'

def getExistingUserInfo(self):

    while True:
        entry = next(self._users)
        uid = entry.pw_uid
        if (uid not in self._uids):
            self._uids.add(uid)
            return entry
