Superman Volume 1 # 174

CLARK KENT'S INCREDIBLE DELUSION!
Clark Kent becomes convinced that his life as Superman has been an hallucination and that he has no super-powers, but it is all the plot of an android duplicate of Superman who wishes to take his place as revenge for being discarded.

SUPER-MXYZPTLK... HERO!
Mr. Mxyzptlk comes to Metropolis as a crime-fighting super-hero and soon makes Superman seem obsolete.