It is common knowledge in comic book circles that Marvel and DC have, over the many years that both giants of the industry have been in existence, ripped each other off. Every Marvel hero (and, in many instances, villain) seems to have either a pre-existing or post-dated DC equivalent - Ant-Man and Atom, Deadpool and Deathstroke, Hawkeye and Green Arrow, the list goes on and on and on.
The myriad similarities in both company's rosters is a product of their close and often intimate relationship over the years. Both companies were founded (and are still based in) New York City, as were a number of other smaller publishers that have gradually been absorbed by one or the other. NYC being the epicenter of the comic book universe meant that every artist and writer worth his salt was based there, and a great number went between Marvel and DC over the years.
This back-and-forth of a number of influential comic book creators over the years cultivated an environment that was accepting of, and almost encouraged copying from the rival to the near-point of plagiarism. Yet, while the practice of ripping-off superheroes from each other became common, less common was the blatant and direct copying of an entire story arc.
Thanks to the MCU, one story that everybody seems to be familiar with now is Marvel's Civil War, though fewer people are familiar with the DC story that it blatantly copied. Mind you, DC (while perhaps the less guilty of the two) aren't entirely innocent either, as you're about to see for yourself. It seems all is fair in love, war and comic books.
6. Secret Invasion Ripped-Off Millenium
Marvel's Secret Invasion (2008):
Who do you trust? That was the tagline for Marvel's Secret Invasion, a crossover story that ran through a self-titled eight issue limited series and spawned a number of tie-ins published that same year. The arc is based on a long-term invasion of Earth by the Skrulls, a hostile alien force capable of shape-shifting that replace a number of Marvel heroes with impostors.
The story begins in the aftermath of the Kree-Skrull War when Iron Man, Mister Fantastic, Namor, Black Bolt, Professor Charles Xavier and Doctor Strange form a secret group named the Illuminati with the purpose of confronting the Skrulls. The plan goes awry when they are caught and subjected to a number of experiments and studies at the hands of their captors, allowing the Skrulls to learn the secrets of Earth's superheroes.
Posing as Spider-Woman, Skrull Empress Veranke leads her double-agents of S.H.I.E.L.D in an assault on Earth that claimed the life of heroine Wasp. Reed Richards is wounded by Skrull Criti Noll (who was posing as Henry Pym) seconds after determining a way to identify the impostors, though he lives to develop a device that allows him to differentiate between real heroes and Skrull invaders. This lead to Veranke's eventual defeat at the hands of Hawkeye and Norman Osborn.
DC's Millenium (1988):
Published 20 years before the strikingly similar Secret Invasion, Millenium was also based on a hostile alien force that were able to plant impostors in the superhero world using their technology, though in this arc the villains were known as the Manhunters, an enemy that the Justice League believed to be dead and gone. Just like Secret Invasion, several supporting characters turn out to be impostors that the JLA have to identify before they can defeat.
The Manhunters began to craft their plan after gathering intimate and detailed intel on Earth's heroes from Harbinger's sphere, and decide to go ahead with their invasion after the Guardians of the Universe (a group who made their debut in 1960 and are not to be confused with Marvel's Guardians of the Galaxy, who appeared 9 years later) announce that they are looking for 10 humans to join their immortal ranks.
Before the search for The Chosen can be completed, the Manhunters have their agents reveal themselves and launch an attack on Earth's superheroes, who are joined by Harbinger in the battle. They defeat the Manhunter agents and then attack their home planet where they fight and defeat Highmaster, mother of all Manhunters.