One Steve Limit/Anime and Manga

Inversions, aversions, and exceptions of s in Anime and Manga include:

A-E

 * Osamu Tezuka had a manga series entitled Adolf ni tsugu ("Tell Adolf"), that featured three characters with the name, set during World War II. One was Adolf Kamil, a Jewish emigre living in Japan, the second was his friend, a Japanese German named Adolf Kauffman, and then... umm... a third one...
 * In Arata Kangatari, both main characters are named Arata. They're generally distinguished by called the one who has a last name (Hinohara) by it. In Japanese, the reader can distinguish between the two in the dialogue by whether or not "Arata" is written with kana or kanji; Hinohara's name is written with the latter, and any character in either world that knows about the nature of the switch will have their dialogue use the correct kanji/kana when referring their respective Arata.
 * Aria the Animation features Alicia and Alice which are the same name (Italian/English). It never was bought up.
 * Baccano! has both a Nick and a Nicholas, as well as a Gustave and a Gustavo and a Goose.
 * Bakuman｡ has two Hattoris, who are apparently not related. Akira Hattori is the main characters' editor, while Yujirou Hattori is Eiji Nizuma and Shinta Fukuda's editor. Yujirou is typically referred to by his first name to keep them separate, and when he calls Akira by his last name, Akira thinks to himself that he has the same last name.
 * The CLAMP metaseries does this very confusingly. There's Played with annoyingly in that everyone with the same name looks exactly alike..
 * Making that worse, the latter pair Head hurting yet?
 * But oh it gets better!
 * And now there's Sakura in their new manga, Gate 7, who doesn't seem to have anything in common with any of their other Sakuras.
 * Den-noh Coil has, as a plot point, two major characters with same-sounding given names, Okonogi Yuko and Amasawa Yuko. They get different nicknames (Yasako and Isako) quickly.
 * Detective Conan uses this in some mysteries for added... mystery. Not to mention they have a huge list of minor characters, so every once in awhile names are going to HAVE to cross with each other. For example, the series' character sheet listed two one-shot chatacters called Hiroki, and there's at least one more not listed there.
 * Hilariously used in a recent case where one suspect was also named Kudo Shinchi (aka the main character.) Status Quo Is God
 * Digimon has pairs of monsters with the same names that are distinguished by their levels—Rapidmon (Armor) from the Digimon 02 movie and Rapidmon (Ultimate) from Digimon Tamers, and Kumamon (Rookie), originally Bearmon, from Digimon World 3 and Kumamon (Hybrid), originally Chakkumon, from Digimon Frontier. When two digimon are different enough but of the same name and level, fans list the year the new one was introduced to draw a distinction such Adventure and Adventure 02s Agumon and Savers' Agumon(2006).
 * Some are fixed by Dub Name Change. There are two digimon named Deathmon who are completely unrelated. The dub names the new one Ghoulmon to distinguish them. Then there are two digimon named Deathmeramon, who are only superficially related. The Dub changes the new one to Skullmeramon.The New Metalgreymon may get the same treatment.
 * In an episode of Excel Saga, the heroine Excel puts up fliers around the city in an effort to find her missing partner Hyatt. In the next episode, one of the fliers is found by two girls named Mikago Hyatt and Excel Kobayashi, who misunderstand and think they're wanted for auditions for a band. These two show up again in another episode as guests on a TV show to sing the Excel Saga theme song.
 * In Eyeshield 21, the Kyoshin Poseidons have two Hiroshis on their team (Hiroshi Ohira and Hiroshi Onishi).

F-J

 * Gakuen Alice has two characters with the same name: Misaki, a teacher and Misaki Harada, a middle school student, both of which are major characters.
 * In Gantz there are Kei Kurono and Kei Kishimoto. Less confusing than other examples by the fact that Kurono is a lecherous teen boy, and Kishimoto is a sweet but depressive busty girl.
 * GetBackers has a Kaoru Haruki, a Haruki Emishi and a Kaoru Ujiie.
 * Ghost Sweeper Mikami has two Orihime; one's a weaver for spirits and gods, the other is the legendary star goddess whose doomed love affair the Tanabata festival celebrates. Still, since they're both practically one-shots (the former, after her first appearance, is only referred to by name once or twice afterwards, and the latter is a one-shot), there's no risk of confusion whatsoever.
 * Hayate the Combat Butler's Izumi and Isumi, are mixed up by fans often, but haven't yet had the issue within the story. Until the story, they apparently had little interaction and even their social groups are separated, so it might never come up.
 * Most of the male characters have similar-sounding names, but given the Unwanted Harem style of the story, they're never likely to come to even meet.
 * Higurashi no Naku Koro ni'' has Jirou Tomitake and Daiki Tomita—both of whom are almost always addressed or referred to by their surnames. Incidentally, they both happen to look similar, though Tomita is just a minor character.
 * Played With in Ichinensei ni Nacchattara. The people who knew Iori as a teenager and see him as a first grader don't suspect anything, because teenagers turning into children is impossible and they assume that "Iori-chan" just happens to have the same name as someone they know. Of course, both Ioris are the same person. Later,  he questions whether or not he ever was a teenager or had just received memories of someone with the same name, a la Swamp Thing. Of course, this is not the case and he gets better.

K-O

 * Kamichama Karin has Kujyou Himeka and Karasuma Himeka. Their names are written with different kanji, though.
 * Koi Koi 7 has two characters named Yayoi Asuka. One is a pink-haired ditz, the other is an eyepatch-wearing silver-haired ruthless type. Once the latter Yayoi is rebooted, she then goes by "Gantai-chan" (literally "Eyepatch-chan").
 * Black Butler has Queen Victoria's butlers Charles Grey and Charles Phipps, also known as the Double Charles. They are usually individually referred to by their last names. Both characters are white-haired and wear white liveries but otherwise look different enough to be easily distinguished from each other.
 * Mai-Otome has two recurring characters named Mikoto, one an Expy of the My-HiME character of the same name, the other is Queen Mashiro's pet cat.
 * Miyuki has two girls named Miyuki. Both are love interests for the protagonist.  Confusion between the two does become a bit of a plot point.
 * Monster has two Martins, two Adolfs, and two Karls. There are also three different Ottos, although one is a story-book character within the series.
 * Nana's two main characters are Nana Komatsu and Nana Osaki. "Nana" as in "Seven", that is. One promptly nicknames the other "Hachi" (Which can mean Eight, and is also based on the famous dog Hachiko due to her clingy personality.)
 * And later on there are two Misatos and two Rens.
 * Narutaru has the major character Akira Sakura, and minor characters Aki Sato and Aki Honda. None of them are ever mixed up with one another, and it's really just as well - especially given Aki Honda's nature.
 * While the Naruto manga alone, as a sheer result of a huge-ass cast with databooks giving practically every character seen for even a moment a name, has only one exception (both one of Danzo's bodyguards and the host of the 7-tailed beast are named "Fu"), if you go multi-media there are several characters with repeated names. For example, both Sasori's puppet armor (and thus the person ) and the Big Bad of the sixth movie/third Shippuden movie are named "Hiruko", a subject of Orochimaru's cursed seal experiments and the main villain of the Three Tails filler arc are named "Guren", a minor filler character is named "Yagura", which later turned out the name of the former Three Tailed beast host and, and both there are two unrelated Fuma Clans (one from a filler arc, one mentioned to be where ). However, the Japanese spellings of these characters names are different. There was a filler character called Jiroubo in addition to the canon Jiroubo. Also Matsuri was used as the name of Gaara's student/fangirl and a girl in Konohamaru's academy class. In the anime, Sabu for some reason had his name changed to Kin, which was the name of one of the Sound Village genin.
 * Neon Genesis Evangelion oddly has Ikari as the last name for two main characters (father and son) and a secondary character whose first name is Hikari.
 * One Piece has a couple people named John: Zoro's old friend and fellow Bounty Hunter Johnny, Marine John Giant, one of the Zombie Generals of Thriller Bark was named Captain John, and the G8 Filler arc features a Marine Vice Admiral named Johnathan.
 * Ace's cover story, where he's hunting down Blackbeard (real name Marshall D. Teach), once has him run into a similar looking but completely unrelated Dr. Blackbeard, who he mistook for Teach, kicked in the face, and was throw into a river by the townsfolk this angered.
 * It has also has two Jones's - Hody Jones and Davy Jones himself.
 * It has also has two Jones's - Hody Jones and Davy Jones himself.

P-T

 * Which Pokémon trainer do you suppose Ash Ketchum respects more? Aaron, the Elite Four member and bug specialist? Or Aaron, the ancient aura-wielding knight whose Lucario he befriended? And don't forget about Drake and Drake, from the Orange Islands or the Hoenn League, respectively. Or Flint, Brock's father, and Flint, a member of the Sinnoh Elite Four and rival to Volkner. Or Bianca, the girl who was frequently impersonated by Latias, and Bianca, the ditzy female rival in Unova.
 * In all four cases, the reason for this is because one character is originally from the games and one is anime-exclusive.
 * In the French games, Brock and Steven are both called Pierre, the latter also being given the last name Rochard. Probably just to avoid confusion, when Steven/Pierre Rochard shows up as a guest star in HGSS, he doesn't give his name.
 * The (in-game) granddaughter of Professor Oak and an (anime-only) elder sister of Misty are both named Daisy.
 * Daisy's name in one of the manga adaptations is inexplicably changed to May, which happens to be the name of the female protagonist in Pokémon Ruby and Sapphire.
 * The sheer number of characters in Prince of Tennis means that names end up being repeated. There are three Hiroshis (Wakato, Yagyuu and Chinen), two Hikarus (Amane and Zaizen), two Kentarous (Aoi and Minami) and so on. Luckily the majority of them are on a Last-Name Basis. (Note also that many of these names are homophones, but written with different kanji.)
 * In Puella Magi Madoka Magica, there's Kyouko and Kyousuke, both recurring characters. This may not look identical, but since "-ko" and "-suke" are gender-specific name suffixes, their name were practically homophonous. So, when a character called 杏子 was introduced after Kyousuke, people thought her name would be pronounced Anko because of this trope. "Anko" remained Kyouko's Fan Nickname.
 * Rave Master has two characters named Musica, because the author liked the name and couldn't decide which of the concepts to use. Also, there were two main characters named Gale; one  and one  . When the latter was named, he was initially assumed to be the former.
 * School Rumble has the Japanese Delinquents Kenji Harima (or, in the Japanese surname-first style, "Harima Kenji") and the American exchange student Harry McKenzie—names which, when pronounced with a Japanese accent, sound nearly identical. Naturally, one of them is often confused for the other in conversation, and Hilarity Ensues.
 * Also, one of the main girls is called "Suou", she briefly dates a boy called "Asou", nobody seemed to care how similar-sounding their names were.
 * SHUFFLE! has two Rins: the male lead and one of the love interests (in her case it's short for Nerine). Sometimes you can tell which character is being addressed by the Honorifics used. For instance, when Sia uses the affectionate term "Rin-chan", she means Nerine; she likes Rin but doesn't know him as well, so he's usually "Rin-kun" unless she's being very serious.
 * In Sketchbook, cat lover Sora encounters a new stray cat and decides to call him Haa. Her brother then points out that there's already a cat with that name around, so she comes up with a new one. She also names all Calico's "Mike", after the Japanese name for Calico "Mikeneko", resulting in "Mike the 2nd", "Mike the 3rd", etc. Eventually neither Sora, nor the cats themselves can remember who's who.

U-Z

 * Averted in Umineko no Naku Koro ni, where the Ushiromiya family's (presumably) human butler Genji's family name ("Ronoue") is pronounced almost exactly the same as the name of Beatrice's demon butler ("Ronove"). This may be significant.
 * The World God Only Knows has two characters named Ryou. One is a delinquent at Keima's school, and the other is Nora's human partner.
 * Subverted in X/1999. One character is named Seishirou Sakurazuka. Another one is named Seiichirou Aoki- and is referred to as Aoki, probably in order to avoid confusion.
 * Your Lie in April averts this at the meta level with the English dub cast, which has two Ericas: Lindbeck, who voices Kaori, and Mendez, who voices Tsubaki. Extending to the similarly-sounding Erika with a K adds a third: Harlacher, who voices Emi.
 * Yu Yu Hakusho has Kurama's little stepbrother Shuuichi, which is the same as Kurama's human name.
 * Yuri!!! on Ice has ice skaters Yuri Katsuki and Yuri Plisetsky, both main characters. In universe, the Russian Yuri is nicknamed "Yurio" by the sister of the Japanese Yuri, a nickname that becomes more popular than his Russian nicknames/diminutives Yura and Yuretchka. Out of universe, fans romanize the name of the Japanese Yuri sightly differently (the form "Yuuri" is the most common in fanworks), or just call them Yuri K. and Yuri P. whenever needed.

Played Straight, despite expectations

 * Suzumiya Haruhi: There's only one Haruhi. When Kyon asks about her in the movie Disappearance, only one character can remember a Haruhi from years ago. This is a name so common in Japan that it doesn't get a One Mario Limit in spite of the series' popularity, but in her world, she really is that special.
 * This just might be justified by what Haruhi is, so she wouldn't want anyone else to share her name.