Characters need to be unique in terms of design, style, and personality in order for them to make a lasting impression. While this is often the case, there have been several cases of character designs overlapping, even if they are from two completely different anime with completely different themes.
There is only a finite number of hairstyles, hair colors, and facial features for artists to use when creating a character, meaning that such similarities shouldn't be all that uncommon. Regardless, it's still up for debate whether these following cases are simply coincidences or direct inspirations of one another.
10 Kallen Kozuki & Akiza Izinski (Code Geass & Yu-Gi-Oh! 5D's)
Code Geass and Yu-Gi-Oh! are by no means similar anime in terms of plot, general aesthetics, or general designs... for the most part. Kallen Kozuki from Code Geass and Yu-Gi-Oh! 5D's' Akiza Izinski have very similar designs, especially when it comes to their face and hair. Although they have different eye colors, blue for Kallen and brown for Akiza, they both share pink hair, styled in very similar ways.
While Akiza Izinski's strength comes from her extremely powerful psychic ability that allows her to materialize and bring monsters to life, Kallen's strength comes from her passion, aggression, and phenomenal mecha-piloting skills.
9 Shanks & Gildarts Clive (One Piece & Fairy Tail)
One of the most commonly known design comparisons in anime due to the popularity of the two franchises, One Piece and Fairy Tail, Shanks and Gildarts have so many similarities that it is almost surprising that they don't share the same voice actor.
They are both older than the main characters, with Shanks being a father figure to Luffy and Gildarts being a father figure to Natsu, and they both have red hair styled in similar ways with light stubble to give them a rough and gritty look.
8 Shinichi Izumi & Yuri Katsuki (Parasyte & Yuri!!! On Ice)
Shinichi Izumi goes on a rollercoaster ride in terms of character development in Parasyte, starting off as an innocent high school student with glasses before his right arm is infected with a parasite named Migi. Yuri Katsuki from Yuri!!! On Ice doesn't go through trauma of that variety, but actually looks almost exactly like a 16-year-old Shinichi. This is all despite the fact that they are completely different characters, pursuing completely different ventures.
7 Mikasa Ackerman & Rukia Kuchiki (Attack On Titan & Bleach)
Appearance-wise, Mikasa Ackerman and Rukia Kuchiki from Attack On Titan and Bleach respectively look almost identical. From the shape of the face down to the chin and the generally negative expression to the hair which is the exact same style if not for some differences in shading.
Aside from Eren being Mikasa's weakness, neither Rukia nor Mikasa tend to open up and discuss feelings with others, generally keeping to themselves. However, they are both highly skilled fighters, with both of them ranking at the top of their respective classes.
6 Hanzo & Saitama (Hunter X Hunter & One Punch Man)
Hanzo is the Hunter who passed the Hunter Exam at the same time Gon and friends did. He's also the man who broke Gon's arm in the process. His overconfidence and sometimes brash personality complement his ninja upbringing, making him extremely formidable.
With his bald head and expressive eyes, he could pass for One Punch Man's Saitama, providing that he wears a serious expression. Otherwise, they'd just be two bald guys, indistinguishable from the rest of the crowd.
5 Takumi Rindou & Maes Hughes (World Trigger & Full Metal Alchemist: Brotherhood)
Despite not being too alike in terms of personality, Takumi Rindou from World Trigger and Maes Hughes from Fullmetal Alchemist: Brotherhood do have a lot in common. They both have black spiky hair, glasses, a similar smile, a scratchy line of stubble, and similar work uniforms. The two of them could be the same person if not for the difference in attitude.
However, in both of their worlds, they are considered to be rather carefree, with Maes Hughes being overly friendly with everyone to the extent that people seem actively annoyed by him at times.
4 Yami & Eve (To Love-Ru & Black Cat)
Konjiki No Yami/Golden Darkness, commonly referred to as Yami, is a main character in To Love-Ru and To Love-Ru Darkness and is a skilled assassin who can change character completely when she undergoes the Darkness Transformation.
In a similar vein of don't judge a book by its cover, Eve from Black Cat is also unassuming and dangerous, with her being a genetically modified clone and bio-weapon. They both have long blonde hair styled similarly at the front and usually sport black get-ups.
3 Sieghart Caeser & Jellal Fernandes (Rave Master & Fairy Tail)
Although Rave Master's Sieghart Caeser and Fairy Tail's Jellal Fernandes have different skin tones, eye color,s and jaw shapes, their similarities cannot be ignored. The blue hair and the mark over the right eye are what make them look strikingly similar.
Sieghart and Rave Master were actually the first results of Hiro Mashima's work as a manga artist, and he would later go on to make Fairy Tail. This explains the blue hair, eye marking, and general character design similarities, but they both are also considered immensely strong and capable of using magic within their own universes.
2 Issei Hyoudou & Tatsumi (High School DxD & Akame Ga Kill)
Issei Hyoudou and Tatsumi are both the main male protagonists of their respective anime, High School DxD and Akame Ga Kill. They both have an almost generic main character look about them with brown middle-length hair in similar styles, with similar facial features apart from eye color.
In terms of personality, however, the two are vastly different. Tatsumi overcame his naivety and arrogance almost immediately once he joined and was established in Night Raid, while Issei would continue to act selfishly. Both possess plenty of courage and bravery when the situation requires them to be so.
1 Feitan & Levi Ackerman (Hunter X Hunter & Attack On Titan)
Hunter X Hunter's Feitan of the Phantom Troupe is one of the evil faction's most dangerous members, and his generally sinister and dark demeanor only emphasizes that. He will often have the lower half of his face covered, focusing attention on his hair and the upper half of his face. Thanks to his glum expression, he could definitely be mistaken for Captain Levi from Attack On Titan.