Why not just tax only the cargo that is unloaded at the port, which would avoid the double taxation problem? It would also place greater revenue in the hands of the consumers that are paying for the shipment of those goods.
Depends. Ideally you want to tax something that's a good proxy for pollution. You don't want to tax trade.
There wouldn't be greater revenue for consumers: the money that pays for the taxes ultimately comes from them.
(Just to mention it: the standard solution for all revenue raising taxation is, "land value tax". Any other tax is only useful as a `sin tax'. Alas, right now we have sin taxes on labour and capital..)
