The time fixed for the hearing shall not be less than 15 days after the completion of the required publication. Not less than 30 days prior to the time fixed for the hearing, a copy of the notice of the hearing shall be transmitted by the secretary of the board of directors to the several boards of supervisors of the counties comprising the district. No county or portion of a county shall be assessed within any funding district for the payment of any amount which has previously been paid by such county into the district treasury upon any amount previously levied within and upon such county.