Kode

Kode was the result of Fitz Kreiner having his personality (and whole state of being) copied into a Remembrance Tank when he joined Faction Paradox.

Biography
Kode was created when Fitz Kreiner was re-remembered throughout the time it took the Anathema to travel to Earth through space. Some of Fitz's personality traits had come through the process, allowing the Eighth Doctor to recognise him as Fitz or his descendant. Like all members of the Remote, he was open to all signals, including the Doctor's TARDIS', which made him more open to the Doctor's cause. Either through these signals, or Kode's wish for the Doctor to restore him to whom he had been, Fitz Kreiner was re-re-remembered using a remembrance tank and the TARDIS's memories of him. Kode preferred not to remain as himself as he would always be left wondering how he compared to the 'original' Fitz. To the Doctor, who assumed the original Fitz was dead, recognised that a man was the sum of his memories, Kode was Fitz in every way that mattered, even if he wasn't the 'original' Fitz. (EDA: Interference - Book One, Interference - Book Two)

Trivia

 * The Eleventh Doctor would also spend time with a duplicate of a companion created from another person's memories after the death of the original in the form of the Auton duplicate of Rory Williams