One of the Superman/Batman arcs involved a group that was supposed to be the Avengers. It was horribly confusing - and that's over and above Mr. Mxyzptlk's involvement.

Image's Supreme had the League of Infinity, based on the Legion of Super-Heroes.

RE: The RECOMbatants and Project Youngblood. I seem to recall Mark Evanier noting that, at that time, he and Marv Wolfman really wanted to do a Titans/DNAgents crossover, but the companies couldn't come to any agreement (or some other details... the man is old, the memory faded.) So they did an unofficial crossover, with both writers very well aware of what they were doing.

We do have the "coincidences" to wonder about, the concepts that seemed too close to be mere coincidence but that premiered almost simultaneously (and certainly too close for either Marvel or DC to have seen published before they did their version)--

The Doom Patrol and the X-Men--Two teams of misfits led by a wheelchair bound genius.  (Premiered June, 1963 and September, 1963, respectively.)

Swamp Thing and Man-Thing--Two scientists changed into swamp men by an explosion in their swamp labs.   (Premiered July, 1971 and May, 1971, respectively--each scripted by roommates Len Wein and Gerry Conway!)

OMAC and Deathlok--One-man-army types always speaking with the computer voice in their head, set in dystopic futures.  (Wouldn't normally associate these two just by sight, but talking to their computer voices is pretty similar!)  (Premiered September, 1974 and August, 1974, respectively.)

Dreamslayer was based on Dormammu.  The rest were correctly guessed.

Hyperion is a well Marvel cannot stop going back to, conjuring multiple versions over the decades. Thor #279 is worth noting as it features the Supreme version versus the Sinister version and arch-foe Emil Burbank, who, due a mishap in both their childhoods, hates Hyperion for giving him hair he cannot cut. The issue is written by Roy Thomas and drawn by Wayne Boring.

Gladiator, of course, is the Superboy knock-off from the Imperial Guard used everywhere around the Marvel Universe as a Superman stand-in. His name has since been given as Kallark. 

The Comet, a drunken, washed-up super-hero who's lost his family, appeared during Carmine Infantino's run on Nova, looking a lot like the Flash. An amnesiac speedster from another universe whose name was something like "Buried Alien" appeared during Mark Gruenwald's run on Quasar. Gruenwald also wrote the Squadron Supreme mini-series that fleshed out the roster and added a bunch of lookalike villains to the picture, and he's the man who "rescued" Wundarr by turning him into the Aquarian. He is, I believe, also the originator of the Grapplers, a Marvel team of lady wrestlers who bear more than a passing resemblance to Big Barda and Her Female Furies.

Batman has a number of Marvel duplicates including Blackwing, the Shroud, and various incarnations of Nighthawk. Wonder Woman may have been an inspiration of sorts for Stan Lee's Lyra and the Femizons, which provided the backstory for Roy Thomas's Thundra shortly afterwards. She's also represented by incarnations of the Squadron's Power Princess. Spider-Man's sometime-foe-sometime-friend the Black Cat bears a strong resemblance to the Catwoman, as does the visual for the Cat and later the Hellcat. The Black Cat was later given "bad luck" powers, perhaps to differentiate her a bit from that other feline-themed, romantically-inclined jewel thief. 

On the DC side, Star Brand and Jim Shooter were parodied as "Sunspot" in the "Legends" mini-series. Batman villains Blockbuster and the Black Spider bear certain visual similarities to the Hulk and Spider-Man and Solomon Grundy has been portrayed a number of times more in the mold (so to speak) of the angry, child-like Hulk than his original monstrous self. It's been suggested that the story of the tiny, cyclopean villain the Thunderer in the original run of Metamorpho was a parody of the Galactus Saga. Well, maybe... Fred Hembeck pointed out that one of the first parodies of Spider-Man ever done was a 1964 issue of Jerry Lewis wherein Jerry becomes the "Fearless Tarantula!" The Inferior Five fought a villainous team of Avengers lookalikes and also introduced Professor Egghead's School for youngsters whose powers didn't represent mankind's future, but rather selected abnormalities we may have possessed at certain points in the past. There was a corresponding team of villains to match the Brotherhood of Evil Mutants as well.

That brings up another supposed coincidence, with the Doom Patrol's Brotherhood of Evil preceding the X-Men's Brotherhood of Evil Mutants by only a short amount of time.

When Roy Thomas created the All-Star Squadron, a small number of Golden Age & WWII era characters where brought back or re-purposed in the Marvel Mold, notably the Tarantula, the Fairytale Fredericks version of Thor, and the patriotically themed Steel. 

Dan Jurgens did an FF knock-off with a team of rocket adventurers returning to Earth and dying from their new super-powers. The leader of that team was later revealed to have survived and has gone on to become the Cyborg Superman and a Red Lantern. Marvel riffed on the FF to create the Hulk villains, the U-Foes. 

Dave Sim's "Cerebus" had a Batman parody called "The Cockroach" who lifted from Jules Feiffer's "Hostileman" strip and later went on to parody Captain America, Wolverine, and the Sandman. Other Cerebus parodies include the Woman-Thing and the Giant Stone Thrunk. Marvel returned the favor by way of Claremont's Cerebus-styled demon S'ym. 

The National Lampoon had an ongoing feature called "Joe" about a young man with largely unexplained super-powers. Later, we met his deadbeat father who looked an awful lot like Superman. Harvey Kurtzman and Bill Elder had fun with their version of S*p*rm*n in their Help Magazine strip "Goodman Beaver..." but if we start listing every Mad Magazine-style parody version, we're going to be here for a while. I will give an honorary shout out however to that same team's "Woman Wonder," a brilliantly twisted satire.

Let's see... who else? Rather than risk damaging Captain Marvel's simpler, more kid-friendly world with a crossover (gasp! Imagine that! Prioritizing the integrity of a feature!) the first Superman-Captain Marvel battle actually involves a lookalike named Captain Thunder. The Captain Marvel comic responded with an everyman-getting-super-powers story featuring a Superman-costumed "Cape-Man." One could argue that every "de-uniquing" character is just a knock-off of the original, but again, the list grows very, very long at that point... And this post is already long enough! 

I will say that Dreamslayer struck me a sort-of Dormammu/Nightmare mash-up character...

Then a few months later, an homage to the X-Men found their way into the main title.

These particular stories weren't worth a second read, but we did get some sweet Alan Davis covers out of them.