Mahou Sensei Negima!'s Kaede Nagase, in addition to the usual illusionary swarm, when she limits her duplicates to four or so all of them are fully tangible and devastatingly effective combatants. Her student and main lead's Rival Kotaro has also shown to be proficient at it, though not to the extent of her ability. Oddly, Nagi, the Disappeared Dad of series lead could do it, while not being eastern or showing any skill in ninjutsu. Rule of Cool indeed.
Negi can also do it, summoning a bunch of doppelgangers when he fights Evangeline. Later on, he turns it Up to Eleven by summoning a thousand decoys. Made of lightning. Just because.
Negi's initial spell actually summons elemental spirits and "clothes" them in a copy of his body (presumably just because his incantation didn't specify otherwise). The divergence from this trope is that the copies are separate beings which only look like him and don't think, act, or fight anything like he does. Since they were pursuing Evangeline, none survived long enough to make this evident, though she selected her defense based on the distinction.
Suzaku, an early Big Bad from YuYu Hakusho, could create seven identical copies of himself, each with all of his strength. In stark violation of the Law of Conservation of Energy, he could then reabsorb those copies to completely replenish his strength, enabling him to fight forever, until Yusuke kills all the clones and damages the remaining clone's antenna, so he can no longer control his aura. As a result the one remaining clone is rendered powerless and possibly dies, though his fate is never actually addressed.
In Dragon Ball and Dragon Ball Z, both this and its sister trope, Doppelgänger Spin, appear occasionally. Some characters, like Tenshinhan/Tien, Piccolo and Cell can divide their bodies into weaker physical copies to employ special fighting strategies. Piccolo also uses it to spar with himself when ther'es no suitably strong partner available.
This is one of the advanced ninja skills in Naruto. All ninjas learn how to make illusionary copies of themselves with the "Bunshin no Jutsu (Clone Jutsu)". Naruto himself uses a much more powerful version, the "Kage Bunshin No Jutsu" ("Shadow Clone Jutsu"), which makes copies which are solid, albeit fragile. This version is also more chakra-expensive, which isn't a problem for Naruto.
There are other solid clone jutsus too, mainly in the form of "X Clone Jutsu", where X is an element like water or sand, with varying levels of similarity to the original. Their disadvantage compared to the Shadow Clone Jutsu is that the user needs to have a supply of the element in question on hand (ie to make Water Clones you need to be near a river or lake and for Sand Clones you need to be near a beach or desert...or do like Gaara and carry a huge gourd full of sand wherever you go). For just a handful of clones, that's rarely a problem. Creating thousands of them like Naruto sometimes does with Shadow Clones, though, is implausible even if you've got a near-bottomless supply of chakra like Naruto.
Pain also has the ability to make copies of other Akatsuki members out of living people, which are essentially the same except with only the percentage of chakra put into it.
In Busou Renkin, the villain Moonface's Busou Renkin, Satellite 30, allows him to create 30 versions of himeself, each one representing a phase of the moon with a head of the corresponding shape. FWI, the new moon one is headless.
Nico Robin of One Piece has worked out how to use her Flower-Flower Fruit powers, which enable her to sprout any part of herself from any surface, to create a full body double.
In Gakuen Alice, Misaki Harada's doppleganger Alice allows her to create multiple versions of herself.
While she's generally more likely to use it for scams than for combat, Ryoko of Tenchi Muyo! is able to create solid "shadows" of herself.
The Alpha Flight character Flashback can conjure up versions of himself from seconds away in time, allowing him to swarm an opponent all by himself. He's not so good at picking foes he can actually beat this way, though, and the downside is, if one of his phase-forms dies, he gets to worry if it's the one from his future.
The mercenary Timeshadow, a former pawn of Apocalypse, has a similar power.
Quantum, a foe of The Avengers, can teleport back and forth so fast that he's effectively two (or more) places at once, allowing him to hit heroes from several directions.
Superman can also do this in a weak form, once playing ping-pong against himself to entertain some kids.
Multiple Man from X-Factor. Especially recently as his clones tend to reflect part of the typical human personality. His libido manifested with a clone and much sex (not with clones) ensued.
The Makuta Bitil from BIONICLE wears a Mohtrek; or in layman's terms: A Mask of Time Duplication. When mentally triggered it summons different versions of the user from different points in time into the present. These duplicates are just as strong as the original was or will be at that point in time. When dismissed, the duplicates will return to the very same moment they left, meaning that no one will have seen them suddenly vanish and reappear. They also lose their memory of what happened while summoned to avoid any whacky time paradoxes. The downside? The summoned duplicates will return to their time with all the physical evidence of the summoning intact, meaning that if you get wounded, that wound will seem to have suddenly appeared on you somewhere in time. And if one gets killed... all version past that one gets erased from time, as if they were never there. So a past version of you gets killed, and you cease to exist; even in the memories of those you were fighting, and all the people you have ever met. Whoops.
Superman foe Riot has the ability to clone himself at will or when attacked. One of Riot's most significant advantages in a fight is that the force of a blow delivered to one duplicate is dispersed across all of his currently-active duplicates; as a result, while Superman could theoretically knock Riot out if he hit a single Riot with a sufficiently powerful punch, with a dozen or more Riots active at once Superman's blows fail to cause sufficient damage to render any of the Riots unconscious.
Films - Live Action
In the film Sky High, Penny Lent, the cheerleader, has the ability to duplicate herself.
Live Action TV
Ninja-theme characters in Toku shows frequently do this.
This normally happens in Super Sentai whenever there's something involving ninjas. It also happens in Power Rangers some times.
Yellow Mask in Hikari Sentai Maskman, who was raised as a ninja, had the Yellow Mask Kage Bunshin ("Shadow Clone")
Zyuranger's Boi (Tiger Ranger) does this when he trains to be a ninja.
Kamen Riders Knight and Decade have this ability when using the Trick Vent and Illusion cards, respectively. When the two fight in the latter's series, each of them use their respective card to fight one another, leading to four Knights vs three Decades.
The tabletop RPG Continuum does this in a way: since all characters are time travellers, they're within their rights to resolve that they'll come back to this moment later on, and watch as their temporal double pops in out of nowhere. The risk is that this means committing to a return into a dangerous situation, and if you see your older self get killed, then that's a pity, isn't it?
One of the Swordmage powers in the supplement book Arcane Power creates three copies of you that you can use as starting points for your other powers, though they all share your action pool.
The first edition supplement book Oriental Adventures also has the Conjuration spell "Body Outside Body", allowing a Wu Jen to create a few clones of himself. Those have less Hit Points than the caster and cannot use spells, but they have all of his martial prowess.
Trickery Devotion, a feat from 3.5, lets you clone yourself 1/day. It's a very frail clone, but still...
Quite naturally, duplication powers are a staple of superhero RPGs.
In the 7th Edition of Gamma World, one of the origins, Doppelganger, focuses on such powers. At 1st-level, you could briefly have three of yourself simultaneously. (It's an at-will power.)
In the Disgaea games, Mid-Boss' ultimate attack, Adonic Fury, utilizes this. He splits himself into four copies and surrounds the unfortunate target to beat the ever-loving crap out of him/her/it. There's also his Super Adonis, which has him rapidly magichange countless copies of himself into swords and rain them down upon his unfortunate targets.
Several other special moves, such as Doppelganger, use copies as well. The makers of the game have commented that the attack is "also useful if you want to vote for something".
In those same games, the "Clone" Geo Panels recreate duplicate versions of whatever is standing on a matching color panel, be they allies or enemies. All of the generated clones are very real (and will invariably attack your party), and retain the stats and Hit Points of the original, but you can't steal any of their equipment.
It seems like every wizard miniboss in Baldur's Gate fights with the mirror image spell, meaning you need to do at least one attack on him per image he creates before you can touch him again. All while he's launching off his own spells. The quickest way to strip this off of him is with a magic missile spell or two, but it's still massively annoying, especially if they use fear or charms on your party (which most do).
Worse is the simulacrum spell, where only one image is produced, but it's got hit points and can wander away from the caster.
In Mega Man Battle Network games, ShadowMan makes clones of himself that each shoot a blast of fire down the row they are on whenever the original does. He can also create a more ghostly clone to appear on your side of the field which attacks you with a sword.
In World of Warcraft, the first boss of 40-man Temple of Ahn'Qiraj (Skeram) (and his 5-man protege in Arcatraz (Skyriss)) uses this ability, although there is a "real" one. Each of his copies must be separated, and the fake ones must be killed before they destroy the raid. The 5-man version in Arcatraz only splits into 2, whereas the 40-man version splits into 3.
As of the latest expansion, Mage players can also perform their own Doppelganger Attack by summoning 3 to 4 copies of themselves able to cast basic spells.
Multiboxers are players who play multiple accounts simultaneously by passing the keystrokes from their keyboard to multiple active WoW installs. When a player does this while playing mage characters, when they are glyphed for Mirror Image and get 4 dopplegangers of themselves, per character, this can be a completely overwhelming sight to see. 5 characters with 4 backups each = 25 total characters. When they all start spamming Frostbolt it looks like Star Wars. Bonus points if they're all gnomes with pink hair.
All of those spells are based on the Warcraft III Blademaster's Mirror Image which summoned 3 clones of the caster. Those dealt no damage whatsoever and were mostly there to confuse the enemy, whereas in World of Warcraft mirror images typically do damage as well.
Jandice Barov of Scholomance is the champion of this trope as she summons no less than 15 illusions of herself midfight. Those deal damage, but TAKE NONE, it is imperative to find the real Jandice (her name is distinct from the images, so she can be found easily) and down her fast.
In Mists of Pandaria, the attack was changed to a Doppelgänger Spin. The images no longer deal damage, and their attack is only to distract you from finding the real Jandice; they can also be destroyed, though they will explode when killed.
Grand Magus Telestra in The Nexus summons duplicates of herself midway during the fight.
In Diablo II, the Amazon and Assassin both have abilities like this. The Amazon can create an illusion of herself and summon a real spirit warrior to aid her; the Assassin can summon a shadow of herself with somewhat reduced skills, or a more powerful version at higher levels with greater skill levels.
Baal himself can do this in the expansion. Thankfully, he can only make one copy.
Recurring Psycho for Hire Yuber from the Suikoden Series has this ability. In terms of sheer damage and utility it's one of the most powerful in the series and appears to be unique to him.
Parsee Mizuhashi summons a copy for one of her spellcards. It's a trap, and retaliates if you shoot it.
During the stage 6 fight with Yuuka Kazami in Lotus Land Story, she frequently splits into two. Not her best idea, as they both take damage.
However, splitting in two also allows Yuuka to fire Master Spark twice at the same time, so it kinda balances out.
Suwako Moriya has a survival card that ultimately has three versions of her trying to kill you. And there's another version of her sunk into the background.
Flandre Scarlet has a spellcard called "Four of a Kind". "Four of a Kind" is the most popular example of this trope in the series.
Mamizou Futatsuiwa possesses a spellcard that generates two copies of herself, which, after attacking for a short time, generate two more copies EACH, and two more later. By the end, you're facing off against ten copies of her before the spell dispels.
Suika Ibuki can subdivide into an uncountable number of smaller versions of herself, to the point that she can take a form that closely resembles a mist (manipulation of density probably helps).
This trope is mostly used in the fight against Dark Samus in Metroid Prime 3: Corruption. She divides into 2 or 3 identical looking copies that are all capable of attacking. However, you can tell which is real in a few ways. You can scan them; the copies register as "Dark Echo" while the real is "Dark Samus". They move pretty fast, so you have to keep your eye on the right one when you stop scanning and go back to the combat visor. Also, there are certain attacks that only the real Dark Samus will use. Unfortunately, they render her invincible for the duration of the attack, and she tends to recall her duplicates immediately after doing them. Also, only one glows under the X-Ray visor.
The Clone spell from Heroes of Might and Magic III creates a duplicate of a unit stack with the same attack power as the original. However the clone is much weaker and is dispelled if it takes any damage.
The fourth part features "Create Illusion" and "Phantom Image", both of which create a semi-opaque unit stack which is the same type as the targeted stack, but with numbers based on the spell's power. Repeated casts stack, you can keep casting Create Illusion on a stack of two Titans to get three or four with each cast... those don't have any spellcasting potential, but are as good at attacking and taking hits as the base monster they were made of. Meanwhile, the fifth part has a spell just like the third one has, creating a same-number copy of the target stack that disappears of one hit, but has a 50% chance to avoid a physical hit.
The Shadow robot in One Must Fall uses shadow copies of itself in all of its special moves. However, to prevent abuse, attacking a doppelanger does minimal damage to the main robot.
There are several Super Robot Wars examples. Zweizergain and the Assault Scouts are prominent examples of machines that utilize this tactic.
Death Metal, the Rank 10 Assassin in No More Heroes, produces two doppelgangers when his health gets low enough. They have as much health as he does at that point, making finding the real one a bit of a chore.
In the obscure roguelike game Steamband, Fu Manchu can create clones of himself in battle. The only way to know which one is real is to kill them.
In the Hordes of the Underdark module to Neverwinter Nights, there is a hall of mirrors where most either give items of various kinds or gold, one creats a mirror you for you to fight. This is rarely a difficult fight, though, as it fails to copy your companions.
Maybe due to a glitch, if you destroy the mirror instead of looking into it, it spawns two copies of your character.
When Dante utilizes his Doppelganger style in Devil May Cry 3, he can create a single duplicate of himself. The duplicate will mirror Dante's movements and attacks, but the player can choose to increase the lag between the command inputs and the duplicate's response — or a second player can take control instead.
One of the bosses in Persona 2 uses a Doppelganger attack as her gimmick. You have to hit the real one, or else any attempts to use magic will result in a nasty counterattack (her copies reflects magic). The solution: fight her on a full moon - the fakes have no shadows. Or watch carefully when she taunts you afterwards.
One of Hanzo's special abilities in Samurai Warriors 2 allows him to create up to three shadow doppelgangers this way. Nene's ability might also count, although they move more like a spin (occupying the same space, instead of spread out like Hanzo's).
Marquis does this in Final Fantasy Tactics A2. He splits himself into five, and strangely enough, all of them have differing abilities. His doppelgangers may know anything from a Flaming Sword all the way to illusion magic that hits the whole field. This happens in a cutscene, so you're effectively just fighting a group of ninja that all happen to share the same name.
One of Konoha's supers in Arcana Heart has her creating a bunch of doppelgangers to pummel her opponent with a 100-hit combo.
Halo 2: The Heretic Leader boss fights alongside holographic clones of himself, whose attacks are just as deadly as the real thing. They can actually take more damage than him.
Great Tiger, in both of his appearances in the Punch-Out!!!! series, uses special attacks where he makes clones of himself to confuse Little Mac. Dodge or block at the wrong time, and the "real" clone will hit you pretty hard.
The last battle in Assassin's Creed I against Al Mu'alim, where he creates a dozen illusions of himself, each of which is perfectly capable of killing you, being the most dangerous fighters in the entire game. If you can figure out which one is the original, you can cause the others to disappear. That's pretty much the entire point of the battle, really.
In the second game Ezio uses the piece of Eden in his battle against the Pope. The copies are different in that they attack independently from you and they don't disappear when you or they are hit. It makes the battle rather easy as you can just wait until he is fighting one of the copies and then attack him from behind and the wait until another copy attacks and he's busy fighting him. Rinse and repeat, Pope defeated.
In Dynasty Warriors: Gundam, the final battle in Domon Kasshu's "Original Mode" path pits him against Master Asia, who uses his powers to create several clones (of varying toughness) of his Master Gundam, and Domon has to fight them simultaneously to achieve victory.
The Legend of Dragoon had the Sandora Elite miniboss, who will create two clones of himself when his HP falls below half. The trick to finding the real one is paying attention to the attacks; the two clones will attack after being created, but since the real one made the clones on his turn, he won't attack.
Xemnas can briefly create clones of himself to attack the heroes during his final fight. This is particularly annoying when he traps Sora in an HP-draining beam and control briefly switches to a Guest Star Party Member - you have to get to Sora and free him, but Xemnas keeps sending clones of himself to stop you.
Larxene's "Absent Silhouette" in Kingdom Hearts II Final Mix can also do this - there's a reaction command where Sora grabs one Larxene, spins her around and throws her into another, fusing them together again.
The Unknown from Birth by Sleep does this too, in case you thought his attacks weren't hard enough to dodge already.
One of the defining characteristics of the Protopet in Ratchet & Clank 2. It can very quickly go from one, to seven, to forty two, and by the time you've killed half of them, the remaining half have probably already spawned the number back to it's upper limit.
Taokaka from BlazBlue has a Distortion Drive called "Almost Becoming Two!", where a transparent doppelganger appears behind her for a short time. Whatever moves Taokaka uses, the doppelganger uses.
Lilith as well. But, while Morrigan's clone is mirrored behind the opponent, Lilith's copy follows her arround and does the same attacks with a delay.
In RuneScape, this is one of Nomad's attacks in the "Nomad's Requiem" quest. He can create three copies of himself. All of the clones hit just as hard as the real Nomad, but when they're attacked, Nomad loses focus and dismisses the copy.
The "Labyrinth" in Vindictus features increasingly powerful doppelgangers as the end bosses on each floor (for a total of 5 encounters).
In Lionheart: Legacy of the Crusader, two of the bosses can summon copies of themselves. In both cases, the original AND the copies (which tend to be weaker) need to be killed in close succession, as the original can summon more copies to replace those the player has already killed, or one of the copies can become the original if the original dies.
In the final boss battle of Spartan: Total Warrior, Ares, the final boss, can split himself in three. The difference between the original and the two copies is that the copies get killed with one hit.
This is pretty much the whole point of Phantom Lancer, one of the heroes in Defense of the Ancients: All-Stars - two of his skills are passive and give him a chance of creating an illusionary copy of himself (that can still damage enemies) with every hit against an enemy (the second boosts the chance and gives his illusions a chance of creating illusions of their own). Sadly, the illusion-count tops out at 8 or so, meaning no army of angry Catfolk charging across the battlefield.
Militsa from Tales of Rebirth does this when she goes up against the party solo, creating four low HP clones that are quickly replaced upon their destruction.
Belius in Tales of Vesperia creates a clone of herself once she reaches half HP. It's barely any weaker then the original, but can be removed by simply reigniting the torches in the arena.
The Golden Swordsman in Tales of Xillia does once after you run down his HP for the first time, then again once you run down the HP of him and his clone, resulting in three clones. Something of an odd case, as the duplicates possess distinct fighting styles based off of those of your party members, and are just as powerful as the original.
If you did not hate him enough alreadyat that point, Flying Fox from Heavenly Sword has a habit of doing this in his second boss fight. While it is frustrating in normal difficulty, as the clones do not go down in one hit and block most attacks very easily, it is far worse in Hell difficulty, where they have a habit of attacking at the same time with attacks whose blocks are mutually exclusive, and they do so while ganging up, making dodging the attacks a gamble as well. Defeating him for good is one of the more satisfying moments of the game.
Halfway through the deathmatch with Mai Hem in Perfect Dark Zero, she starts summoning clones of herself. Unlike her, they don't "derez" (in the words of TRON) when defeated, and you can pick up their weapons and ammo.
Ra's Al Ghul in Batman: Arkham City and Copperhead in Batman: Arkham Origins are both shrouded by copies in their boss fights, but they're not actually using this technique. Rather, in both cases, Batman is under the effects of hallucinating drugs.
In one plotline of Sluggy Freelance, Doctor Schlock assists Bun-Bun in a fight by firing a gun loaded with countless plush Bun-Bun dolls. As a result, once Bun-Bun is knocked out, the dolls remain, obfuscating his opponent's efforts to finish the real Bun-Bun off.
In 8-Bit Theater, the heroes end up trapped in a void where causality does not exist. Their solution? Use that lack of causality to create an army of themselves and break out.
Nanase of El Goonish Shive has a "decoy summon spell." It is a partially sentient puppet that can be controlled directly or behave based on the user's mental state at the time of summoning.
Homestuck: Aradia builds an army of her doomed time travel clones in order to fight her session's Black King. They don't do much good against Jack, though.
Fellow Hero of Time Dave Strider is also fond of creating time clones through stable time loops to fight alongside him. His aren't much good against Jack either — Dave's original self dies fighting him this way.
In Trinton Chronicles has Aurora who can make clones of herself to run around and perform multiple attacks against a single target.
Red vs. Blue has this effect whenever there's a Stable Time Loop, usually surrounding Agent Wyoming. For Church, it allowed him to figure out the solution to the time bomb crisis. Wyoming had been using it for much longer, and knew how to use it. When Tucker told Church that Wyoming had been putting them through such a time loop, Church remembered his experience (and doppelgängers), and realized Wyoming must be using this trope. Naturally, they collided with an army of Wyomingstwo seconds later.
He showed up again in Justice League and got the drop on the Flash, who was used to Mirror Master's Doppelgänger Spin and wasn't expecting the "illusions" to throw a punch.
Ben10 has an alien form called Ditto which can split itself into multiple copies, but was otherwise roughly equal to a human child. Its weakness was that all of its copies shared damage; if one was attacked, they all felt it. In Ben 10 Alien Force, he gets a new form named Echo Echo, which has this power along withsonic attacks and minus the damage-sharing.
Experiment 344 (Dupe) in Lilo & Stitch: The Series can make physical copies of anything, but the more copies are made, the weaker they all become.
As part of his ghostly abilities, Danny can duplicate, but it's an often difficult task for him. He managed to succeed in the last season for a brief amount of time. Meanwhile, Big Bad Vlad's been able to do this since his intro.
Nabu from Winx Club may have this ability, but it is unclear whether the duplicates are solid or not. In a fight against Darcy, he was able to attack all of her illusion copies this way.
Also, Darcy herself can do this in addition to the normal Doppelgänger Spin, but she's limited to three copies who surround the opponent to attack from multiple direction, and hitting one will dissolve the copies and knock Darcy out. This invariably terrifies Musa into helplessness, as the first time Darcy did it the attack nearly killed her trough sheer pain.
The Tick animated episode "The Tick Loves Santa" features a villainous thief in a Santa Claus costume who aquires the ability to create multiple clones of himself by absorbing electricity. Connecting himself directly to a generator at the hydroelectric power plant, he spanws hundreds of clones almost instantly.