1. Heart of Darkness &The Secret Sharer
Joseph Conrad
Heart of Darkness &The Secret Sharer

2. Doppelganger “A ghostly counterpart or double of a living person”

3. The Double
A device whereby a character is self-duplicated, as in the case of Leggatt and the Captain in Conrad’s The Secret Sharer, or divided into two distinct, usually antithetical, personalities, as in Stevenson’s The Strange Case of Dr. Jekyll and Mr. Hyde, which objectifies the internal struggle of good against evil (primal self).

4. Doubling by Duplication
A twin or physical duplicate

5. Doubling by Division
The counterpart represents one facet of the character, customarily spiritual or psychological in nature
The counterpart can personify one’s demonic counterpart
Alter ego – Dark and light