The Central Government may, in such cases as it thinks fit, certify that a person with respect to whose citizenship of India a doubt exists, is a citizen of India; and a certificate issued under this section shall, unless it is proved that it was obtained by means of fraud, false representation or concealment of any material fact, be conclusive evidence that person was such a citizen on the date thereof, but without prejudice to any evidence that he was such a citizen at an earlier date.