Cast Speciation

""We don't need two stretchy guys down there!""

- John Stewart, Justice League Unlimited

A looser adaptation of the Five-Man Band. Generally speaking, it's bad form to have two major crew members on the show who essentially perform the same job (unless you have a multi-team Heroes Unlimited situation). That would lead to inevitable comparisons between the two, and probably lots of flame wars.

So any character who starts mirroring another too closely will have a different personality which affects the outcome of their work, or will be given a noticeably different job. One way to do this is to make the two rivals, competing to be the best at what they do. In this case, usually one is more friendly and laid-back and the other is more serious and stoic.

Does not apply to Evil Minions who sometimes come in faceless dozens, but does for, say, Battle Butlers or Dragons.

Occasionally, executive pressure or plain bad writing will force a character to not ever evolve past their original role, or instead force a new character into it. In the latter case, the original's days are numbered.

Can be avoided from the get-go by an Economy Cast or lampshaded by an Evasive Fight Thread Episode. This trope falls purely within the Rule of Drama. In real life, some degree of redundancy is generally considered a good thing, so that a group isn't crippled if one member with a specialized skill is taken out the picture.

Not So Similar is the super trope. Compare Divergent Character Evolution, which happens gradually and isn't as deliberate.

Anime & Manga

 * Sailor Moon's Minako originally starred in her own manga, Codename Sailor V, before being absorbed into the series, creating the odd situation where the derived character (Usagi) ended up being the lead. Minako has since undergone Divergent Character Evolution twice: a more outwardly capable but loopy character for the anime, and a straight serious character for the Live Action Adaptation.
 * Pretty much all her incarnations treat her as a veteran, and occasionally showcase her greater experience at being a Magical Girl than the rest of the Senshi, and except in the anime. The manga and live-action show especially shows her being serious and knowledgeable; the anime suggests that not being alone has loosened her up from the more serious personality she had in earlier seasons, but shows her being able to pull magical girl stunts (like leaping onto send-floor balconies from the street outside the house) while in civilian form.
 * Pokémon tends to use this within the main characters' groups of Pokemon, despite the preponderance of Poor Predictable Rock elsewhere in the series. Occasionally, though, Ash will have two different Pokemon with similar powers and movesets; these are usually distinguished by personality.
 * The series also had no other main characters with electric Pokemon until Dawn got a Pachirisu.
 * May and Jessie briefly owned a Wurmple each at the same time, though this was likely intentional, as it provided an opportunity to showcase its multiple evolutions.
 * And May eventually ended up getting both a Bulbasaur and a Squirtle--though unlike Ash's, hers eventually evolved.
 * Dawn captured an annoyingly powerful Buizel, despite already having the water type Piplup and not even being a battler, while Ash didn't have any on his team. Cue rumors that she would eventually trade it to him.
 * Usopp in One Piece was the crew's general inventor and mechanic up until the introduction and joining of Franky, a shipwright who rebuilt himself as a cyborg. This was largely resolved (after Sogeking joined the crew) by emphasizing Usopp's role as a sharpshooter and also by having him focus on personal tools and inventions where Franky is more heavy duty with arming the ship and keeping it in condition. They're also drastically different in personality, Usopp being a Lovable Coward while Franky is more Hot-Blooded.
 * This is also built into the setting. Two people can't have the same Devil Fruit power at the same time - the fruit won't regrow until the last person who ate it dies.
 * Happened a lot in Naruto. Even though for the exception of Elemental Powers and inherited bloodline techniques anyone can learn anything, each character has between 1-5 special moves that no one outside of their clan or direct Master Apprentice Chain is ever seen using.
 * The most notable example is Sakura/Ino. Both had similar personalities (if anything, Ino was a more extreme version of Sakura), but by being each other's rivals, they became characters in their own respective rights. Character development further separated them.
 * Naruto and Kiba also get this treatment; they have similar personalities (again, Kiba is described as being "even wilder than Naruto" by way of introduction), but have very different power sets in order to differentiate between the two. Later on, they become more separated.
 * Completely Averted in Dragonball. Every Character has the exact same powers, with only one or two special moves differentiating them. But by the end of the series, everyone has either taught each other their unique moves, or developed something that essentially does the same thing. Whoever has the highest Power Level is the best in speed, power, endurance and Ki attacks.
 * In short, besides color and hand motions, there's virtually no difference between a Kamehameha, Gallick Gun, Final Flash, or Masenko.
 * The only exception to this rule was when USSJ Trunks was stated to be more powerful than Perfect Cell but too slow to hit him.

Comic Books

 * As noted above, there's been a running gag in The DCU over the years about the Justice League's two "stretchy guys," Plastic Man and Ralph "Elongated Man" Dibny, who have the same powers and similarly light-hearted personalities. (The Elongated Man was created as a Captain Ersatz of Plastic Man, whose creator was unaware that DC had acquired the rights to Plas) DC separated the characters by emphasizing that Elongated Man was also a brilliant detective, and exaggerating Plastic Man's fun-loving side to the point of outright lunacy (at one point Plas says about Ralph, "Nice guy, but someone left the door open and his personality just bolted"). DC seemed to be doing away with the confusion by making Ralph darker and moodier, arguably to the character's detriment, before concluding this character arc by having him die in a Heroic Sacrifice. And more recently, . Which is ... quite different indeed.
 * The JLA used the rule of "no two people can have the same power" to keep Hawkgirl/Hawkwoman out of the League for over a decade. Her husband eventually put his foot down about it. Of course, under that rule (as Scott Tipton once put it), the entire League should consist of Superman, Green Lantern, the Atom, Wonder Woman (only for the whip of truth) and possibly Batman (the Atom's already pretty damn smart, but not in the same way).
 * The whole Plastic Man/Elongated Man thing is lampshaded in Batman the Brave And The Bold, where the two are fighting baddies together while arguing which one of them Batman likes more. In the end, Batman shows up and says that he doesn't really like either of them.
 * Thing is, EM/Plastic Man don't have the same abilities. Elongated Man is a "stretchy guy who happens to be a genius detective-type". Plastic Man basically IS plastic, and can control/reshape every molecule in his body. Plus being nigh-invulnerable and seemingly immortal (3000ish years old and still going strong). Batman himself referred to Plas as the most dangerous being on the planet, so...
 * Back during the Silver Age, Superman and the Flash were equally fast. Modern comics have made the Flash much faster than Superman to balance out Superman having a lots of other superpowers besides just running fast.
 * Specifically, they were so closely matched that they actually held races (at least three of them; the first two were ties, Flash won the third... though by the end of that one, they were literally-and-I-use-this-word-correctly crawling and Batman or, for that matter, Snapper Carr could have beaten either of them).
 * The Legion of Super Heroes used to have an explicit rule that every member had to have an unique power; they've loosened up on this, although they still mostly abide by it in practice. (It's a particularly artificial limitation in the Legion's case, as many of their members have powers shared by everyone on their planet.)
 * And Karate Kid, who doesn't even have any powers of his own. When the Legion was going to veto on him, he showed them that while he doesn't have any super powers, he can literally kick ass in about a thousand different ways. The Legion then decided to change their minds and induct him.
 * Of course, once you actually join the Legion, the first thing they give you is a flight ring - so apparently there is one power it's okay for everyone to have.
 * In the X-Men verse, it has become formulaic for every team to have a telepath, powerhouse, energy blaster, and/or one member who won the Superpower Lottery.
 * ...And one who lost the lottery, either via Blessed With Suck or What Kind of Lame Power Is Heart, Anyway?.
 * ...if not a telepath an animalistic mutant... at least before Wolverine Publicity flew out of control.
 * In the Fantastic Four both Reed Richards (Mr. Fantastic) and Ben Grimm (The Thing) are highly intelligent and honored scholars and scientists, but everyone likes to focus on Ben's physical strength rather than his brain and the result is that the Thing very often comes off as Book Dumb.
 * Most of the time when Reed is going off on his Sesquipedalian Loquaciousness (usually explaining whatever Deus Ex Machina he's cooked up) The Thing's problem isn't with what he's saying not making sense, but simply taking too long to say it when there are more important things to be done. This leads to him losing his patience and yelling "THE POINT, STRETCHO! GET TO THE POINT!"
 * Not to mention the fact that Ben is often seen dumbing down what Reed says for other people.

Film

 * Done in the X-Men movies (the comics... not so much). According to the features, some characters were never used for precisely this reason. At least before the character flood in X-Men 3.

Literature
"My immediate task was to differentiate clearly between Novinha's children when the reader first encounters them. I sat there in the room I shared with Gregg, assigning some immediate and obvious trait to each of the children that would help the reader keep track of them. Oh, yes, Olhado is the one with the metal eyes; Quara is the one who says outrageous things after long silences; Grego is the violent one; Quim is the religious fanatic; Ela is the weary mother-figure; Miro is the eldest son, the hero in the others' eyes. These "hooks" could only serve to introduce the children--I'd have to develop them far beyond that point--but having found those hooks, I had a plan that would let me proceed with confidence."
 * Somewhat averted by playing it straight in the Drizzt Do'Urden novels. Of the five main characters (six counting the mascot), four are skilled melee fighters. It's broken up by having one of them being far and away the best technical fighter; one of them being monstrously strong; another being a tough, tactical group-fighter; and the fourth usually doing ranged support. Later, one leaves, and another shows an interest in magic, while a third gets older and starts attending as much to his political duties as his military ones.
 * The Weasley twins of Harry Potter are wisecracking pranksters with Twin Banter, but Fred is a bit crueler and George is a bit more sensitive.
 * In the introduction to Speaker For The Dead, Orson Scott Card discusses how he accomplished this:

Live Action Television
"Bree: (to Katherine) Here's the thing you need to understand about me and my friends. We each have our niche. Gabrielle's the glamorous one, Susan's the adorable one, Lynette's smart, Edie's ... Edie, and I'm the domestic one, the organizer, the one who knows that there are three tines on a dessert fork, and the one who gets teased for that. That's who I am. And that's also who you are."
 * The last season of Angel put a second ensouled vampire champion on the same show; they went the rivals direction with it and gave Angel, who previously thought of himself as unique, a bit of an identity crisis.
 * Of course, Angel and Spike have had a sort of rivalry since Season 2 of Buffy the Vampire Slayer, in which they vied for fellow vampire Drusilla's affections.
 * Heck, the flashbacks in various episodes show that they've been that more or less since Spike got turned.
 * In Buffy the Vampire Slayer, the split between Buffy and Faith can fit this trope.
 * Same with Kendra and Buffy, with Kendra being a very by-the-books Slayer and using more gymnastics in her fighting style. Also between any two Watchers.
 * While Giles is made fun of because of his British Stuffiness, he's a lot more laid back compared to Wesley (at least before his character development), as well as having a dark past. Gwendolyn Post, a rogue Watcher, is also seen as incredibly stuffy even compared to Giles and Wesley.
 * Willow and Tara, both witches in the Scooby Gang, were set apart by their usage of magic. As Willow grew in power as a witch, she used her powers more often, whereas Tara would only use her magic for support or in dire situations. Willow also had the benefit of being a skilled computer hacker, but she used those skills less as the show went on.
 * Heroes does a lot of this, and follows the trope by making the pair have very different personalities. Peter good, Sylar evil. Claire good, Adam evil. HRG good[ish], Bob evil. Matt good, his father evil. Oddly enough though, Nathan and West are very similar people, the only differences being their ages, Nathan does his domineering sneakily whereas West does it to Claire's face, and West isn't meant to be seen as domineering.
 * Lampshaded in the 4th-season Desperate Housewives episode "In Buddy's Eyes":


 * Lost: Two characters can contact the dead, but comparisons are avoided by the fact that one is a Deadpan Snarker while the other is...Hurley. Also brought up in the plot, as Hurley compares his powers to Miles'.
 * Their powers are also substantially different -- Hurley often finds dead people walking up to him and striking up conversations, whereas Miles can hear a dead person's last thoughts if he's near the body.

Toys

 * The Transformers franchise has this in spades in order to sell more toys. No omnidisciplinary scientists here - the original series' Autobots had a theoretician (Skids), a metallurgist (Swerve), an engineer (Wheeljack), and more. Sadly, the fiction often treats this as an Informed Ability.

Video Games

 * Some players of the superhero MMORPG City of Heroes may avoid having more than one person on the team with the same powers or even the same class to avoid redundancy, especially as a team can only have up to eight members. On the other hand, there are cases when such redundancy may be desired if the superpowers have effects that can easily be stacked on a foe, such as with status effects when used against highly resistant foes.
 * In other MMOs, this is where hybrid classes truly shine, especially in World of Warcraft where players can now take two specializations. For example, if an offensive-casting or melee Druid joins a party that already has strong DPS, she can be the healer or Tank instead, depending on what's needed more.
 * In Mega Man 9, Mega Man lost his slide and charged shots so that he could play more like he did in the first two games. His lost abilities were instead given to Proto Man, who has the aditional ability to block enemy fire while jumping. These differences were retained for the next game, which also included Bass, who can dash and has no Denial of Diagonal Attack. Mega Man, however, does have an advantage over them by being the only character capable of using the Bonus Bosses' weapons.
 * Slightly different, but fits here: The original Fire Emblem featured multiple Palette Swap Player Character units, including the identical-portrait Saji and Maji (Bord and Cord in English). Remakes of that game and sequels with the same cast thankfully changed all of that (Bord and Cord have different tunic styles and are looking different directions now).
 * Touhou has always featured Reimu, a shrine maiden, who is tasked with hunting youkai and preventing them from causing trouble. Terminally selfish and lazy, Reimu sees these duties as an annoying chore, and puts little effort into her training, instead relying entirely on her ridiculously powerful talents and family tools (and she originally hadn't even bothered to learn how to use either of those). Further, she's a staunch Nay Theist; as a shrine maiden, she should be establishing good relations with the gods so that they'll loan her their power, should be encouraging the people's faith in the gods so they have more power to give her... but Reimu simply can't be bothered, no matter how many youkai mock her about it. Reimu outright believes that effort is not rewarded.
 * SO, when a second miko was introduced to the setting, she was almost completely different. Sanae is dearly devoted to (and very friendly with) the two Physical Gods of her shrine and glad to work for them, whether in personal service, promoting faith among the people, or hunting youkai. Sanae was born with incredible power like Reimu, but instead of random human talent, her talents are divine in origin, as one of her goddesses is also her ancestor (though Sanae was left in the dark about this until a later game); her own diligence in training her powers actually earned her the status of "deified human" from people praying to her as well as her goddesses. Finally, Sanae was born and raised in the normal, modern world outside of Gensokyo, and so brings a very different perspective to the crazy inhabitants and events of that land.

Western Animation
""If you're a Super Friend, being able to fly is like being able to break a graham cracker along the line... All the Super Friends who can't fly can just hop in a little supercar or something invisible that flies... Regular people can't fly, but they can kick Hawkman's pigeon ass as soon as he lands.""
 * The Land Before Time has built up a number of secondary characters over the sequels, one of the most popular being Ali, a girl who is the same species of dinosaur as Littlefoot. However, despite her popularity, she has not shown up more than twice and when a guest character was elevated to main character for the TV show, Chomper was picked instead. Admittedly he was popular in his own right, but the likelihood that the writers did not want two characters of the same species in the main group was probably a factor.
 * Unfortunately averted in the old Super Friends series, as Seanbaby notes. Hawkman and Hawkgirl could fly, but so could every other hero on the show (even Flash when the animators were forgetful).