to be fair, there is much in Marvel that is a direct ripoff of DC.
Also to be fair, there's a lot of DC that is ripping off Marvel, and parts of both that are ripping off other comic companies or other media entirely.
For instance, it's not like most of the illiterate people today would even remotely be aware of this, but Green Lantern is basically a straight-up rip-off of the Lensman series (which DC openly acknowledge when they named one of their Green Lantern characters Arisia, which is the name of one of the planets in Lensman). Batman is basically Zorro, Superman was blatantly based on a pulp story written a few years earlier called Gladiator (and nearly every superhero comic you've ever read exists solely because Superman established the genre), Thor is coming straight out of real world mythology (with a weird Kirby twist), the original version of the Flash is pretty literally just the god Hermes wearing colorful long-johns, Green Arrow is straight-up Errol Flynn Robin Hood with a modern day twist, and so on.
And then there's stranger things like Swamp Thing/Man-Thing, where both companies basically created near-identical characters at almost the exact same time with no real evidence that either company was aware of the other's plans in advance. Or the twisty incestuous spiral you get when you consider that X-Men was probably a rip-off of Doom Patrol, which was in turn sort of a blatant attempt to make a Fantastic Four-ish sort of team, which was itself incredibly similar to the Challengers of the Unknown (which was also created by Jack Kirby), which in turn was sort of a Doc Savage/pulp adventure rip-off itself. Or the 437 different Captain America-esque comic characters that all popped up around the same time because the zeitgeist of the era was perfect for them.
People who whine about either company stealing from each other (or anyone else) are sort of missing the forest for the trees - very little entertainment these days is all that "original" (name a popular story or comic you enjoy, and I can probably name at least five things it's stealing its ideas from). And if the homage/affectionate parody/rip-off is interesting in its own right, who the fuck cares?
Hell, even if we accept the idea that X-Men was 100% copied from the Doom Patrol, I can also safely say there has never been a single moment in the entire history of the universe when I'd actually want to read a Doom Patrol comic or see a Doom Patrol movie rather than an X-Men one. If a "copy" does what it intends to do better than the "original" it's based on, then it's by definition the better product. And cross-pollination of ideas can absolutely lead to better products being created by everyone.