(o) In the event of a repair of a leak in the water supply system to a dwelling unit, the landlord shall determine as accurately as possible the amount of water that was measured on the submeter for the dwelling unit as a result of such leak, after a review of the billing records for the dwelling unit and consultation with the licensed plumber repairing the leak. The landlord shall then determine the amount of the bill for the billing period in which the leak occurred that was attributable to such leak and reduce the bill to the tenant by that amount or, if such bill has already been paid, grant the tenant a rebate in that amount; provided, however, that with regard to any leak about which the tenant knew or should have known, the landlord shall only be required to reduce the bill to the tenant, or to grant a rebate to the tenant, by or in an amount attributable to the water usage measured on the submeter as a result of the leak between the date the tenant gave notice to the landlord of the leak and the date the leak was repaired.