One of the significant events in the Superboy series during the Silver Age was the arrival of Mon-El on Earth. He became something of the poster boy for the imperfection of the Boy (and later Man) of Steel, as both Superboy and Superman were unable to solve his lead-poisoning problem and rescue him from the Phantom Zone. Indeed, we learned in the Legion of Super-Heroes series that Mon-El was only able to join the corporeal world 1000 years in the future.
But in fact, Mon-El's story was at least partly swiped from an earlier Golden Age tale. Here's Mon-El's ship crashing, from Superboy #89 (June 1961):
And here's how the story looked, way back in Superman #80 Jan-Feb 1953):
Superboy deduces that the stranger on the rocket ship is his older brother:
For the same reason that Superman claims Hal-Kar as a sibling in Superman #80:
The middle section of each story is quite different; in the Mon-El intro, we learn that Superboy begins to suspect that Mon-El is a fraud, even though he displays very similar abilities to a Kryptonian on Earth. In the original Superman realizes that his older brother has slightly weaker super-powers to his. However, the explanation for how each ended up on Krypton is very similar:
Large elements of Superboy's Older Brother were clearly swiped from Superman's Older Brother