When World War I broke out, it was not uncommon for navies on both sides to commission civilian ships for military use, either as transports or warships. One sneaky move was to arm civilian ships, especially passenger liners, to trick the enemy. Such ships could use the element of surprise to attack an enemy who believes the ship is nothing more than an unarmed civilian vessel.
In 1914 the German Navy armed the 18,000-ton, 613-foot long passenger liner Cap Trafalgar with two four-inch guns and six 37mm auto-cannon. In addition the Cap Trafalgar was disguised to look like a similar British Cunard line passenger liner called the 19,524-ton, 650-foot long RMS Carmania. The idea was that the Cap Trafalgar could approach British merchant and supply convoys with little suspicion of being a German warship. When the convoy least expected it, the Cap Trafalgar would open fire and destroy the convoy.
On September 14th, 1914 the now-SMS Cap Trafalgar was discovered off the coast of Brazil by the converted British passenger liner, now auxiliary cruiser HMS Carmania of the Royal Navy.
Whoops. Talk about small world.
The Carmania was likewise armed to raid German merchant convoys, and was disguised as the Cap Trafalgar just as the Cap Trafalgar had been disguised as the Carmania.
For 90 minutes the two doppelgangers battled each other in a gunnery duel, often at ranges no more than 200 yards. During the battle the real Carmania took the most hits and suffered heavy damage. After being struck with 72 shells her bridge was completely destroyed and she sustained hull ruptures under her waterline. However the real Cap Trafalgar suffered mortal damage and sank into the ocean, taking 51 German sailors with her. The rest of the German crew, 279 men altogether, were captured by the British.
Carmania however returned to passenger liner service, being refitted in 1923. In 1932, she was sold to Hughes Bolckow & Co., and scrapped at Blyth.