Cal. Elec. Code § 3109

If any military or overseas voter to whom a vote by mail ballot has been mailed and which ballot has not been voted by him or her returns to the county in which he or she is registered, or for a military or overseas voter qualified pursuant to paragraph (2) of subdivision (b) of Section 321, to the county in which the applicant’s parent or legal guardian resided when the parent or legal guardian last lived within the territorial limits of the United States or the District of Columbia on or before election day, he or she may apply for a second vote by mail ballot pursuant to Section 3014. The elections official shall require him or her to sign an authorization to cancel the vote by mail ballot previously issued when it is returned to the county elections official. The elections official shall then issue another vote by mail ballot to the voter, or the elections official shall certify to the precinct board that the voter is eligible to vote in the precinct polling place of his or her residence.