An English duke in the 18c would not be the public owner or directly involved in managing an import business. There were no formal prohibitions against titled nobility being involved in trade in England (unlike France, where it could jeopardize one's tax privileges to be actively engaged in commercial activities that weren't about agriculture or real estate) -- but the social prohibition was strict. If the duke is going to be making money in the import/export business, he's going to do it through passive investment or by going through third parties, and he will never participate directly in the operation in any way that doesn't allow for plausible deniability should anyone outside the firm catch wind of it. (People might whisper in scandalized tones about where does he get all that money, but they can't pin anything on him and all that money is a bit delicious, isn't it?)
Either that or he's a complete social renegade who doesn't give a shit what anybody else thinks of him, & doesn't care about its impact on anyone else in the family either.