[1] When the Navy was expanded once more at the outbreak of the Napoleonic Wars in 1803, Duncan joined HMS Narcissus in the Eastern Mediterranean and served in operations in the Aegean Sea and off Egypt.
With Narcissus ordered back to Britain in 1804, Duncan joined the first rate HMS Royal Sovereign under Admiral Sir Richard Bickerton as a lieutenant and it was aboard this ship that he received news of the death of his father.
The death of such a senior officer was marked by numerous letters of condolence, including one from Horatio Nelson which offered command of the small brig HMS Bittern, whose captain Robert Corbet had been taken ill.
[1] Returning to the Mediterranean, Duncan was promoted to post captain in 1806 and took over the sixth rate HMS Porcupine in 1807, sent to serve in the Adriatic campaign by operating off the coast of the Kingdom of Italy.
[1] Duncan was then given the large frigate HMS Imperieuse and commanded her in the Western Mediterranean off Toulon and Tripoli, seizing a number of French merchant ships.
Raiding the Northwestern coast of Italy, Duncan succeeded in capturing numerous ships and destroying large quantities of supplies in fortified harbours before Imperieuse was forced to undergo extensive repairs in Port Mahon.