Right, you can never return a cube to a city it's already been to.
I don't think this is correct, and if it is its not correct for the reason stated. The "no traveling through same city twice" would not come into play with the scenario the poster poses if you go devenport, melbourne , smithton, or vice versa.
Further, There is nothing in steam rules or the rules of this expansion that say you can't do such a shipment, so if it's not allowed its something that wasn't expressed in the expansion rules.
However, doing so by itself would not increase shipping length as the poster suggests because the rules do state that the ferries don't count as links. There are still many valid reasons to want to pay $4 to ship through both ferries to melbourne though.