In any case where an ordinance is enacted or a regulation adopted by a district board relating to the same subject matter and applicable in the same area as an existing ordinance or regulation of a related governmental subdivision for the district, the district ordinance or regulation, to the extent of its application, supersedes the ordinance or regulation of the related governmental subdivision. In any case where an area within a district is served for any district purpose by a system, works, or facilities of the district, no system, works, or facilities shall be constructed, maintained, or operated for the same purpose in the same area by any related governmental subdivision or other public agency except as approved by the district board.