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.see more on Duel Masters
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.see more on Mermaid Melody Pichi Pichi Pitch
'Dive!!' Borrows From 'Free!'
One look at Dive!! an it's obvious that it was trying to be the new Free! It focuses around a diving club that's about to be closed down due to financial troubles, and then dovetails into the interpersonal issues between its members, many of which are based on bonds formed in early childhood. It attempts to court a straight female audience with a combination of beautiful characters and emotional depth, but where Free! succeeds, Dive!! fails. Most of its characters are pretty one note, the art and animation isn't nearly as good as what Kyoto Animation can come up with, and the story doesn't stand out on its own.
If Shinchiro Watanabe never stops reusing his own material, that will be a boon to the anime community. Samurai Champloo borrowed elements from Cowboy Bebop - this makes sense because they were directed by the same person. Both series use music as a motif, both follow a group consisting of two men and a woman on a journey, both feature protagonists with remarkably similar personalities, and both feature a combination of standalone episodes and an unfolding plot.
Watanabe obviously cannot be accused of ripping off his own work - instead, he's an example of someone who effectively used what was working about his own show to create something equally compelling.see more on Samurai Champloo