Let's face it - not every anime is wholly original. Actually, there are plenty of anime that borrowed from other anime - or at least, they sure seem like they did.
These aren't necessarily anime ripoffs. There are only so many ideas in the world - after all, how many anime are there where two high school sweethearts fall in love, or where a hero saves the world? What truly makes a series noteworthy isn't whether it's wholly unique, it's how it handles the material its chosen. Some of the shows on this list are clearly influenced by other series, but that doesn't mean that they don't have their own merit. Guilty Crown and Code Geass are both great shows - they're just really really similar.
That being said, there are definitely some items on this list that weren't trying to put their own spin on material they cared about, and were instead just trying to cash in on a trend. Monsuno: World Master may be entertaining to some, but it's hard to argue that it's not a lackluster Pokémon clone.
If you like anime where everything is determined by card games but don't want to have to watch hundreds of episodes to get there, you may be tempted to watch Duel Masters instead of Yu-Gi-Oh! However, you also have to be cool with watching a rip-off that just flat out isn't as good. Shobu Kirifuda is trying to become a master at Kaijudo, a game in which aspects of other dimensions are brought to Earth via cards. It's kind of a cool premise if you ignore the part where it's exactly the same thing as Yu-Gi-Oh! or the part where it's completely lacking in meaningful character development. Duel Masters is an okay show to pass the time with, but it would have been a lot more memorable if it had tried to differentiate itself from Yu-Gi-Oh! in any way.
Both Attack on Titan and Kabaneri of the Iron Fortress tell the story of humans trying to survive in a heavily fortified community against terrifying man-eating beasts that may have once been human. Ultimately, the protagonists enter a perilous struggle to defeat the enemy and protect their communities from harm.
Kabaneri of the Iron Fortress was technically created by the same studio that produced Attack on Titan, but Kabaneri is an original production while Attack on Titan is based on a manga by Hajime Isayama. It seems like Wit Studio was trying to replicate Attack on Titan's success, but they didn't quite manage to do so. Kabaneri has its own charms, sure, but it's no titan of the anime industry - pun intended.
Sword Art Online may be one of the most popular anime ever made, but that's not because of how original it is. Before Sword Art Online came out, there was .hack//Sign, which tells the story of an MMORPG player named Tsukasa who is unable to log out from the game. SAO takes that concept and expands it to the entire player base. While each anime is markedly different in both tone and story, this key element can't be denied. In fact, it's possible that every anime that involves being transported to a game-based reality, including Log Horizon, Overlord, and more, owe their roots to this 2002 classic.
Take Sailor Moon, subtract the space theme, add mermaids and music, and you have Mermaid Melody Pichi Pichi Pitch. It's a magical girl show in which the blonde pigtailed protagonist must keep her true identity a secret. Her fellow magical girls look suspiciously like members of the Sailor Scouts, too. It's a cute anime if you've already seen Sailor Moon and can't get enough of that wholesome magical girl vibe, but it's not exactly a show that one would call unique.