That’s usually the hero’s reason for having a secret identity, so giving it to the villain is an interesting flip-up. Also, it’s always amusing when Jonah accuses someone in a costume of secretly being Spider-Man in disguise!
Amazing Spider-Man (Vol. 1) #13
Published: c. April, 1964?
Cover Date: June, 1964
“The Menace of… Mysterio!”
Writer: Stan Lee
Artist: Steve Ditko
Inker: Steve Ditko
Letterer: Artie Simek