Section 7. Spirit, opium and salt imported from any port in British India, and protected by the certificate of

7. Spirit, opium and salt imported from any port in British India, and protected by the certificate of an officer empowered in that behalf by the Governor General in Council or the Local Government, are chargeable with only the amount, if any, by which the duty leviable thereon under the second schedule hereto annexed exceeds the duty shown by such certificate to have been already paid in respect thereof. The amount, if any, paid to the Government as the price of such opium or salt is not duty within the meaning of this section.