The concept is not original with superheroes; Sherlock Holmes had his Moriarty, the crime-creating mastermind to his crime-solving genius. Going back further, God found himself in constant battle with the fella with the horns. We've already talked quite a bit on this blog about Bizarro and
SuperMenace, two memorable DC knockoffs of Superman.
But there were many other dopplegangers. Green Lantern had Sinestro:
Sinestro was a renegade Green Lantern who had abused his power and had his ring and lantern taken away by the Guardians. At some point he acquired a ring that emitted a yellow light and thus was invulnerable to Green Lantern's own beam.
Hawkman had the Shrike:
The Shrike was an interesting double in that his powers were similar to Hawkman's (although the lightning-shooting wings were a significant difference) but his origin resembled that of another DC hero:
Instead of landing in Kansas, this rocket landed among a grudge-bearing tribe of Indians who dreamed of the youngster restoring them to glory, which led to the Shrike (initially) being a criminal, stealing back ancient treasures.
Well, if a doppelganger works, then why not a doppelgang?
The Legion of Super-Villains was a twist on the Legion of Superheroes and specifically the original three members of that group, who were similarly seen voting down Superboy on the famed cover of Adventure #247. This particular concept for a cover (which looks like a game show) was also used in Action #296, and Jimmy Olsen #87, making it something of a doppelgang itself.
Over at Marvel, the Frightful Four were a similar but different version of the Fantastic Four. The Justice League of America had an even more similar opponent in the Crime Syndicate of America of Earth-3.
Two of the more interesting duplicates of the Silver Age were Amazo and the Composite Superman. Amazo debuted in Brave and Bold # 30. He was an android who had been given the powers of the members of the Justice League of America (not counting Superman). But World's Finest #142 introduced a character who topped even that:
The Composite Superman was Joe Meach, a failed high-diver who after being saved by Superman, was offered a position at the Superman Museum as a janitor. But instead of being grateful, Meach resented his lowly status. So when a freak bolt of lightning hit the statues of the Legion members at the Superman Museum and gave him virtually limitless power, he used it in an evil way:
Fortunately his powers turned out to be temporary as is his memory of the experience and so he ends up back where he started, pushing a broom.
Oddly enough, a similarly menial position was the origin of a reverse doppelganger, The Leader. While working a mindless job at a chemical research plant, he was exposed to gamma radiation that amazingly stimulated his brain, making him a being of great intellect but little physical ability, the exact opposite of the Hulk as he mentions here:
Some other famed doubles: Crimson Dynamo/Iron Man/, Titanium Man/Iron Man, Gas Gang/Metal Men. Many superheroes had some sort of real duplicate created by a ray or magic or red kryptonite, so we'll ignore those. I don't see a real Reverse Spiderman or Reverse Daredevil or Captain Russia out there, and while you can argue Loki as the anti-Thor, I don't buy it. That's more a personal rivalry that doesn't really imply similarity in powers. For the same reason I'd argue that Dr Doom is not Mr Fantastic's double, nor is Baron Mordo the equivalent of Dr Strange.
Atom battled a few small villains but I don't think any of his villains really mimicked him beyond that. There were of course several Kryptonian criminals who appeared in the Silver Age as doubles of Superman or Supergirl.
Any others?
Update: Kyle points out Professor Zoom, the Reverse Flash. He was in the original post, but somehow an editing mistake left him out. Good catch!
Update II: Here's a different take--multiple androids with the combined powers of the Avengers:
