Christine Chapel was an android duplicate of the Human Starfleet nurse of the same name created by Doctor Aaron Brown in 2268.
In that year, an android duplicate of Captain James T. Kirk of the USS Enterprise replaced his counterpart on Tranquility VII and took command of the ship. The Kirk android then ordered the Enterprise to Midos V on the pretext of searching for missing colonists. In the process, the search parties - including Chapel, Spock, Lieutenant Hikaru Sulu and Ensign Pavel Chekov - were captured and replaced with androids.
The original Kirk was able to escape Tranquility VII and made his way to the USS Hood, which had been taken over by the androids. He and a group of the surviving Human officers were able to destroy the androids and retake the ship. After getting a full list of the replaced Enterprise crewmembers from Lieutenant Nyota Uhura, Kirk beamed over to a ship and gathered Montgomery Scott and the rest of the engineering crew to retake the ship. The android versions of Kirk, Chapel and the others attempted to escape but Kirk was able to track them to the shuttlebay and trick them into destroying a shuttlecraft. The resulting explosion destroyed the androids. The Enterprise then returned to Midos V and Kirk rescued the original versions of Spock, Sulu, Chekov, Chapel and his other crewmates. (TOS novel: Double, Double)