"Stamp Duty" stamps are fiscals. These are also known as revenues and are used to pay taxes and fees. Since 1978 remaining stocks of PNG postage stamps were overprinted with "Stamp Duty" and used as fiscals.
Stamp Duty stamps are not listed in the Michel catalogue, but after Michel number 354 there is a foot-note about these stamps. Specialists are using the Barefoot catalogue.
Subsequent you find illustrations of all Stamp Duty stamps known to date. Current market prices are available here.