CA Rev & Tax Code Section 4990

On discovery that any property is assessed by the same taxing agency more than once for the same year, after payment of all charges justly due on the property the county assessor or the person having custody of the roll shall certify the facts to the board of supervisors. The board of supervisors shall then order the auditor to cancel the other charges and assessments by an entry on the margin of the roll and, if carried there, the delinquent and current roll.