This is a pretty common plotline in the Superman universe, with side characters (invariably brought in as special guest stars) being accused and ultimately proven not to be Superman when they get attacked by his villainous enemies and Supes has to show up to defend them.
A few examples include Superman #145 ("The Secret Identity of Superman!")
Superman: Flight to the North (1955)
Arriving in Metropolis to seek his fortune, the clueless Sylvester
Superman answers a Superman classified ad for the "real" Superman and
before long has been hired by a woman named Marge to deliver a lemon
meringue pie to her fiancé, Steve , stationed at a remote Air Force
weather base in Alaska. Meanwhile, gangster Leftover Louie has wagered
$25,000 that he can convince his schoolmate Marge to bake him a fresh
lemon meringue pie, even though she can't stand the sight of him.
Inevitably, these two plot strands are intertwined, as a hopelessly
confused Steve welcomes the vacuous Sylvester, a gun-toting Louie, and
the honest-to-goodness Superman into his tiny snowbound shack.
Superman #9 ("Secrets and Lies")
Etc etc etc.