During Infinity War the cosmic villain Magus created evil duplicates of Earth's super heroes because he wanted to fuck with them. Or maybe was he trying to take over the universe. Nah, I'm pretty sure he was just being an asshole. Most were destroyed or killed during Infinity War, but Spider-man's doppelganger was brought back to life by Demogoblin to be his pet. At some point Doppelganger wandered off from Demogoblin (this is why we have leash laws people) and ended up being the pet of Carnage's girlfriend Shriek. Then Carnage killed Doppelganger because Carnage tends to do that. Doppelganger has recently appeared in a Carnage mini only to be immediately killed off again. No explanation was given for his resurrection.
I love Doppelganger. Yeah I know he has all the personality of a rabid dog and was in some pretty shitty stories. But look at him. That is a fucking amazing character design. He has segmented eyes, a big crazy symbiot character type mouth, he has big ass claws, eight limbs and he shoots barbed webbing. He's like Spider-man if he was designed by Clive Barker. In the 90's everything in comics was trying way too hard to look "EXTREME" and "SUPER BADASS AND KEWL" but Doppelganger actually did look badass. He looked fucking horrifying. Especially with the right artist. I remember one artist (although I don't remember who or what issue) made the webbing pattern on his look sort of like scales. It's almost upsetting they wasted such an beautifully twisted character design on something that just growled and tried to bite your face off. But when you look like that what else are you going to do? If only they would make a Marvel Legends toy of him.