Page 966. " when I thumbed through that book containing the second part of my history "

   Don Álvaro Tarfe is a nobleman from the 'Avellaneda' author's unauthorised sequel to Don Quixote, who befriends the delusional hidalgo, only to leave him at the Toledo madhouse.  In an ingenious twist Cervantes does not simply oppose his rival's fictional world with his own but amalgamates the two - the unsettling knowledge that there is a doppelgänger of Don Quixote out there somewhere, with his own Sancho Panza, is offset by the legal oath Tarfe swears that Cervantes' duo are the genuine article.

Online copy of Avellaneda's 'Continuation of the History and Adventures of the Renowned Don Quixote de La Mancha', in an altered French version, translated by William Augustus Yardley (1784)