A game rule that disallows you from using multiples of an object. This can apply to individual units ("you can only play one copy of Emperor Evulz") or groups of units ("you can only use one of the five Power Crystals"). How strict this rule is varies — sometimes you can bring multiple copies as long as only one is active at a time, and sometimes you can only bring a single copy.
These rules exist for various reasons. A common one is balance, as it keeps you from spamming powerful effects or abusing ones that would be overpowered in multiples. They also keep complexity in check by limiting the number of complicated effects in play. Moreover, it's useful for effects that are easy to understand on their own, but would be confusing in multiples — what happens if you get a Potion with two copies of the effect "if you would get a Potion or a Scroll, you instead get one of each" active? Another common justification is flavour, as it doesn't make sense to let you use two copies of a unique individual or to have a character be willing to fight for you when you're already allied with a rival they despise. Lastly, the fact that you only get one copy of something makes it feel more special and forces you to make it count.
Some of these rules are so strict that all players can only use one object in total. Typically, these mechanics serve to give the players something to race for or fight over and are justified by them representing a global effect such as changing where a battle is taking place. Another common variety is applying it to playable characters in multiplayer modes where you are otherwise free to pick whoever you want (compare Asymmetric Multiplayer). This makes sense with respect to lore, but can be annoying if two players share a favourite or someone has to get stuck with a Low-Tier Letdown. Most modes that do this are Co-Op Multiplayer; competitive ones tend to allow Mirror Matches.
The last type of Uniqueness Rule is the single-use effect, which challenges players to time its use well. This is the game equivalent of It Only Works Once, though it's usually limited to once per battle/match rather than once for the entire game. There's also an even less strict variation that only disallows you from using it multiple times per turn — this is often used to shut down infinite loops or just keep the effect from being spammed. Notably, these effects are often enforced by game mechanics instead of having "only use this once per game/battle/turn" spelled out.
Sister Trope to Arbitrary Headcount Limit. Sub-Trope of Cap. Super-Trope to Mutually Exclusive Power-Ups. Compare Breakable Weapons, One Bullet at a Time, One-Hit-Point Wonder, Only One Me Allowed Right Now, and Unique Enemy. Also compare Conservation of Ninjutsu, where multiple copies can exist, but at a cost. Not to be confused with There Can Be Only One, which is for long-term plotlines where a group of characters is slowly whittled away, usually by killing each other, until only one remains. Also avoid confusing it with Unique Items, which is when an item is one-of-a-kind in-universe to show its importance, even if that's unreasonable.
In order to qualify for this trope, an example needs to have some gameplay significance. Purely cosmetic uniqueness like "each player needs a different playing piece so that you can tell them apart" doesn't count. Also, many Tabletop Games with vastly different playable characters or factions (e.g. Villainous) only include one copy of each for logistical reasons — it'd be too expensive and impractical to print duplicates just to allow Mirror Matches. Please avoid listing these.
- 7 Wonders doesn't allow you to build two buildings of the same name. This keeps players from monopolizing resources too much and keeps science balanced (as you'd otherwise be able to get absurd Set Bonuses).
- Abandon All Artichokes:
- The carrot, eggplant and rhubarb compost themselves as part of their effect, ensuring that you won't get to use them again. They all have strong effects the game doesn't want you to abuse.
- The carrot has the powerful effect of letting you compost it to compost two artichokes. However, it comes with the unique drawback that you can only do it if it's your only action that turn (you are otherwise free to take as many actions as you want).
- Bang! allows a player to hold one piece of equipment of a given name and only one type of weapon. Playing a new weapon will discard the previous one. Additionally, only one Bang! card may be played per turn (outside of a special ability).
- The Castles of Burgundy: Most tiles can be built in multiples, but you can't have two identical buildings in the same region. This makes it harder to get multiples of the same effect and limits your options for adding buildings in the late game.
- The game has historically had the rule that a pawn could only be promoted to a captured piece, which makes it impossible to have more than one queen at a time. As the queen is a powerful piece, some people disliked the idea of having two at once. However, this rule has since been removed, and promotion is unrestricted by which pieces have already been captured.
- Castling is a special move that moves both your king and a rook, and is only available if neither of the involved pieces has ever been moved (among other restrictions). This means that it's only usable once per game and can't be repeated even if the king and rook somehow return to their original positions.
- In Clue: Discover the Secrets, each character has a one-time power that needs to be used carefully.
- In Everdell, your village can only contain one copy of critters and constructions with the "unique" property. This sometimes has a thematic justification (it makes sense that you can only have one King or Queen), but more often than not it just serves to keep you from spamming cards that would be too good in multiples. For the Innkeeper and Crane (which can be discarded to play respectively a Critter or Construction at a discount) in particular, it's a form of Anti-Hoarding — you can play them and hold onto them, but this means that if another one shows up in the Meadow, you won't be able to snag it...
- Though players can create multiple mercenaries of the same type (much easier in the Digital Tabletop Game Adaptation), only one of each type can be in the party at a time.
- Mercenaries cannot equip more than one of the same item, though Exact Words are in play—for example, while you can't equip two Major Healing Potions, a Major Healing Potion and a Minor Healing Potion is okay.
- Certain actions will cause you to Lose the card they're printed on, preventing you from recovering that card when resting.
- Most Small Items, and certain weapons or armor, are "consumed" upon use, rendering them unusable until the end of the scenario, though certain cards or items can restore them for another use.
- In Happy City, your city can't contain two buildings of the same name. This keeps you from taking multiple copies of one of the affordable "dwelling" buildings.
- Living Forest: The Kodama Expansion Pack adds six types of Kodama-specific Protective Trees. For balance reasons, you can only buy one of each type — not only would they be very powerful in multiples, but there are only two of each type regardless of player count, so it wouldn't be great if one player could grab both.
- Just One: For each word to be guessed, a given single-word clue may only be provided once (including variations of a word). When cluegivers duplicate a word, those get erased.
- The Munchkin expansion "Munchkin Cheats" includes the "Blatantly Cheat!" card which allows a player losing a combat to win it, take the treasure(s), and go up a level. The card has a rule on it that it must be taken out of the game after use, as opposed to the rest of the deck, which is placed in a discard pile that can be shuffled back in or dug through by players.
- The Quacks of Quedlinburg: The titular witches in the Herb Witches Expansion Pack give players access to a once-per-game power.
- Race for the Galaxy doesn't let you build two identical developments. (All the worlds are different, even if a few are Separate, but Identical.)
- In Shōgi, players are not allowed to drop a pawn on any file on which they have another unpromoted pawn already in play. This makes it impossible to create chains of pawns that protect the pawn in front of them.
- In Warhammer and Warhammer 40,000, you are generally only allowed to field one of each named character in a game. The same character can appear in opposing armies, however.
- 13th Age has a rule that each Player Character has "One Unique Thing", which can be basically anything that isn't too game-breaking and that the DM allows.
- Apocalypse World and most games Powered by the Apocalypse have a rule that stipulates that only one copy of a playbook can be in the game at any time. This is justified by a form of enforced Cast Speciation, where a player with the Hardholder playbook is not a Hardholder but the Hardholder in the context of the story.
- Dungeons & Dragons:
- In 3.5th Edition, a character can't take the same feat more than once (unless the feat description says otherwise). Third Edition also had a rule that the same bonus from multiple sources doesn't stack — wearing a Ring of Deflection +3 and having the Protection From Evil spell cast on a character both gave a Deflection bonus to Armor Class, but only the higher one would count.
- Dungeons & Dragons 5th Edition:
- In order to limit the power of player characters compared to previous editions, 5e characters can only be attuned (linked to; a character can only attune to three magic items at a time) to one copy of a magical item to prevent things like attuning to three copies of the best ones that the character can afford in order to stack their benefits.
- Also in 5e, in a related idea, while there's nothing preventing there from being more than one of the same AOE spell on the battlefield at the same time, if the copies overlap, a creature will only be affected by one copy (make one saving throw, take damage once, etc.), not both.
Trading Card Games
- Exaggerated in Altered. Each Unique card is truly unique: only one digital copy exists. In formal play, it can only be used by one person at a time, and they only get to use one copy. While the owner of the digital copy can order extra physical copies via the print-on-demand service, those are only usable as proxies or for informal play — it's the digital collection that determines what you can put in your deck.
- Chaotic: Certain cards are classed as "Unique" meaning only one copy of a card with its name is allowed in a deck. This rule extends to cards with different versions (I.e. Maxxor and Maxxor: Protector of Perim) as well. Certain pieces of Battlegear are classed as "Legendary", meaning you may only have a single Legendary card in your deck.
- Magic: The Gathering:
- The legend rule: you can't control multiple copies of a legendary permanent on the battlefield. This rule has changed throughout the years; at one point you could only have one copy in your deck, and there was also a time where both players could only have one copy on the battlefield in total. The main justification for this rule is flavor (these cards portray unique individuals, items, and places), but in some cases, it's also a balancing mechanism. Some cards interact with this rule, such as Leyline of Singularity making every nonland permanent legendary, and Mirror Gallery lifting the rule.
- The now-lifted planeswalker uniqueness rule, which says that you can only have one planeswalker of each planeswalker type. Currently, planeswalkers are just treated as normal legendary permanents.
- The retired "World" supertype shows up on some symmetric enchantments and brings the restriction that only one World enchantment can be in play at a time. If you play a new one, the old one goes away. Flavor-wise, they represent mutually exclusive global changes to the battlefield.
- In the Commander format, originally known as "Elder Dragon Highlander," each card in your deck has to be unique. The only exceptions are basic lands and cards with the very rare effect of explicitly allow you to go above the normal copies-per-deck limit. (Hence the "Highlander" part: To quote the Tag Line, "There can be only one.") Moreover, you can only have one commander (who originally had to be one of the five Elder Dragons, but can now be any legendary creature) unless the cards explicitly say otherwise.
- The Vintage format almost never bans cards, instead restricting them cards to let players use a single copy of them in a deck.note This gives players a chance to use very powerful cards without making things as broken as they'd be if you could go up to 4 copies.
- The Companion keyword lets you start with a creature outside the game, available to be put in your hand if you pay 3 generic mana, as long as you meet the deckbuilding requirement. This keeps you from using multiple copies of the same companion, which would lead to balance issues (there's no reason not to run the theoretical maximum of 4 if your deck meets the restriction anyway), as well as removing the silliness of trying to meet two different companions' deckbuilding requirements at once.
- Certain activated abilities and triggered abilities have the clause that they can only trigger once per turn to keep them from being used too much. A tapping cost can also be used as a soft "only once per turn" restriction, though there are quite a few ways to get around it.
- Some spells exile themselves as part of their effect, all but ensuring that you won't get to use them again. Generally used for effects that could lead to infinite loops or otherwise be obnoxious if repeated.
- In the My Little Pony Collectible Card Game, a player cannot control more than one Unique card with the same name at a time. This is normally restricted to named characters and one-of-a-kind items, but there's one card that gives the "Unique" tag to all cards in play.
- Netrunner: Only one unique card of the same title can be active at a time. Duplicates are immediately and unavoidably trashed.
- The Awesome, but Impractical Shining Pokémon, as well as their successors Pokémon Star, could only be used at one copy per deck.
- Prism Star cards are powerful cards with the restriction that you can only have one of each Prism Star card in your deck, and if it would normally go into the discard pile, it goes into the Lost Zone instead, ensuring that you won't get to use it again. They also come with the risk of your sole copy ending up among your Prize Cards, which makes accessing it awkward at best.
- Ace Spec cards are so powerful that you can only have a single copy of one of them in your deck. You have to both pick a card carefully and use it wisely.
- GX attacks are powerful attacks with the special rule that you can only use one of them in a game, so you have to make it count. VSTAR Powers have a similar restriction.
- There can only be one Stadium card in play at a time. An existing Stadium card is discarded when a new one comes into play.
- Players can only play one Supporter card per turn. This rule was introduced to Nerf powerful Trainer cards by keeping players from playing multiple powerful ones per turn.
- Inverted by the Star Trek Customizable Card Game, released in 1994. Unique characters — say, Jean-Luc Picard — had no indicators, whereas generic Red Shirts like 4th-season extra Ens. McKnight had that four-diamond icon on her card name to indicate she was not unique. The game's manufacturer, Decipher Inc, apparently learned from this experience; the Star Wars Customizable Card Game, released in 1995, The Lord of the Rings Trading Card Game, released 2001, and STCCG's own Second Edition, released in 2002, all played this trope straight, using uniqueness markers on the special guys instead of generic markers on the extras.
- Limited cards. You can only have one copy of each limited card in your deck. This is the harshest restriction a card can get before it's outright banned.
- Players are limited to having one Field Spell each in play. Early in the game, when Field Spells were Symmetric Effects representing mutually exclusive Geo Effects, it was only possible to have a single Field Spell Card active on either side of the field.
- Malefic monsters have the effect that you can only have one Malefic in total on your field. The field spell Malefic Territory changes it to "you can only have one Malefic of each name", however.
- Some effects are limited to only being usable "once per duel", or, less severely, "once per turn". The former limits powerful effect. The latter does that as well and is often used as a way of avoiding infinite loops.
- There's a Continuous Trap called There Can Be Only One which prevents both players from controlling more than one monster of each type on the field. There are also other similarly restrictive Continuous Traps like Rivalry of Warlords and Gozen Match, although the latter focuses more on the monsters' attributes rather than their types.
- Yu-Gi-Oh! Rush Duel has Legend Cards, which are cards from the original Yu-Gi-Oh! deemed to be very powerful by Rush Duel standards and have a "LEGEND" icon at the top left of their artwork. You can only play one Legend Card of each type (Monster, Spell, and Trap) in a single deck.
- Illuminati: New World Order being a world domination game, only allowed one copy of any location or person to be in play at one time. After all, you can't have two different secret societies controlling the Pentagon at once! Notably, this did not apply to Illuminati groups themselves - if multiple players were using the same Illuminati, the in-game justification was that they were opposing factions engaging in an Enemy Civil War.
- Dragalia Lost:
- You couldn't have two variants of the same adventurer in your party setup. Each party member has to be a different character. Of course, this only applied to your setup, there was nothing stopping players in coop from having a team of all Cleo.
- Even if you get multiple copies of a wyrmprint, you couldn't equip multiples of the same print to a single adventurer. This extended to dominion prints where there were two different versions of some prints, only allowing you to equip one of the versions to an adventurer.
- Genshin Impact: In co-op, while all players can use the same character in the overworld, each player is required to use a different character from one another when entering domains.
- Kingdom Hearts III only allows you to carry one Kupo Coin at a time. So while you can buy another should you die during a battle and use yours up, you can't be saved multiple times in one battle, so the player has to make their revival count.
- Path of Exile: Many unique jewels have restrictions on how many copies of them can be placed in the skill tree. Many of them are limited to one or two to curb stacking abuse or because more would be redundant. Some other limited jewels include Timeless Jewels, which dramatically modify passive skills around it and are limited to one of any kind, and the Grand Spectrum jewels, a set of unique jewels which have increased effect for each Grand Spectrum jewel socketed and are limited to three.
- The Splitting: There are item duplication puzzles involving taking items between the real and mirror world. However, you can only carry one at a given time, as you already have some cleaning fluid that you need to duplicate later.
Card Battle Game
- Card City Nights:
- You can put only one Legendary card in your deck (out of eight possible ones)
- Rare cards are limited to one copy each per deck, but there is no restriction on your total number of Rare cards.
- Hearthstone: Heroes of Warcraft:
- Most cards can be run at 2 copies, but you can only use one copy of each Legendary, which makes drawing them less consistent.
- You can play as many cards as you can afford. In contrast, your Hero Power can only be used once per turn. This serves to make it less spammable, as it's always available.
- "Highlander" cards have extremely powerful effects but have the restriction where your deck cannot contain any duplicates when they're played. Normally this restricts them to a no-duplicate deck. However, you can play a deck that expects to draw all of its cards very quickly, letting you activate the cards after all of your duplicates have been drawn.
- The Duels game mode starts players with a 16-card deck, with no duplicates allowed in deckbuilding at all. Inverted with the bucket system, where you can pick 3 more cards to add to your deck after every game. Not only can buckets include duplicates, it can break the normal 2-card limit and the 1 Legendary limit, letting you have any number of a specific card if you're lucky.
- MARVEL SNAP allows no duplicates, period, unless the duplicate comes from a card or location effect causing you to draw a card from your opponent's deck, or a card can create duplicates (e.g. Sentinel, which respawns itself into your deck when played).
- Downplayed in Slay the Spire. While you can pick up as many copies of a card as you want (assuming you can find them), your score gets the Highlander bonus if your deck contains no duplicates outside of basic cards.
- While in Yu-Gi-Oh! Forbidden Memories you can have up to three copies of any given card, the pieces of Exodia are the exception; you can only have one of each, and the card description specifies it to be the case.
- Yu-Gi-Oh! Duel Links has a much harsher Limited list compared to the regular game, as cards are put on different Limited tiers (from 1 to 3), and you can't have more cards from a tier than its number - whether or not the cards are different. (For example, you can have two copies of one card from Limited 2 or two separate cards, but not both.) Typically, multiple cards from archetypes deemed too powerful in the current environment are put on the same tier, rendering it either impossible or much more difficult to use the deck's key strategies.
- In Dragon Ball Z: Budokai Tenkaichi 3, the only multiples of a character that you can put on the same team are characters that there are canonically more than one of, such as Cell Jr.
- Borderlands series: In addition to the 2-4 ranged weapons, players can equip only one of the following at any time, though they can carry as much as they want in their backpacks, provided they have space:
- In Borderlands: one shield, one class mod, and one grenade mod.
- In Borderlands 2: one shield, one class mod, one relic, and one grenade mod.
- In Borderlands: The Pre-Sequel!: one shield, one class mod, one Oz kit, and one grenade mod.
- In Borderlands 3: one shield, one class mod, one artifact, and one grenade mod.
- In Tiny Tina's Wonderlands: one melee weapon, one magic spell, one ward, one amulet, one ring, and one armor.
- Deep Rock Galactic: The host of the server has the option to prevent class duplication when a mission is underway, preventing players from joining as a class that someone else is already playing as. It's a means to force the team to have at least one of each of the playable classes and their gear.
- Overwatch: The game developed a couple of uniqueness rules over time to help with balancing issues in competitive play:
- During the beta and at launch, it was possible to actually play multiples of the same hero in the same match, leading to memes of seeing 6 Winstons leaping on one character or 6 Bastions destroying the other team. This was soon removed and each team could have only 1 of the same hero on it, though this did not extend to hero classes, allowing for a team to run multiples of the same class (tank, damage, support).
- In the transition from Overwatch 1 to Overwatch 2, role-queue (which enforced a 2-2-2 team composition across the 3 classes) was changed, such that each team could only have a single tank hero, making what once was a 6v6 into a 5v5. This was done to better match the player base (tanks were the least played role in Overwatch 1) and to avoid some of the more controversial metas like "Double Shield", which were made possible by the presence of two tanks on the same team.
- In Perfect Dark's Combat Simulator, each player is only allowed to deploy one Laptop Gun as a sentry weapon at any time. Attempting to deploy a second Laptop Gun as a sentry gun will result in the first deployed gun destroying itself.
- In AFK Arena:
- You aren't able to have more than one copy of the same character in battle at once, even if on a different team in the same battle. This also applies to using Awakened versions of characters at the same time as their normal versions, though those can be on separate teams. CPU opponents are, of course, not bound by these restrictions.
- You can't upgrade more than one copy of a given character past Legendary+ rank.
- Fate/Grand Order likewise disallows the use of two copies of the same Servant from your own roster in battle. A duplicate Servant can only be set from the support list. Additionally, any Servant can only be set on one of a player's three Normal and Event support lists. If Normal Support One, for example, has Mash in the Extra Class slot, she cannot be set in the Extra Class slots of Normal Support Two or Three.
- In Puyo Puyo!! Quest:
- While the game doesn't stop you from forming a team of nothing but your favorite character, Combination Bonuses won't be granted past duplicate characters, encouraging variety in team composition.
- Teams with duplicate characters are not allowed to enter Technical Quests.
- In Monster Sanctuary, a game that mainly consists of 3v3 battles, you can only use one Ultimate Skill per turn.
- EverQuest II: Items listed with the Prestige tag usually end up being a Best-In-Slot item for any given expansion. The game restricts the player to only wearing 3 Prestige items to prevent them from becoming even more overpowered.
- Final Fantasy XIV:
- Green gear acquired from dungeons and blue gear acquired from raids typically come with the "Unique" modifier, meaning that a player cannot own two of that item at once. This also means that it's impossible for a player to equip two identical rings that have the Unique modifier.
- In Triple Triad, you are only allowed to play one copy of a card in your deck under most circumstances. In addition, players can only have one 5-star and one 4-star rarity in each of their decks, preventing someone from simply stacking high-rarity cards to overpower a less experienced player. However, the NPC players are not subject to the same rules.
- Kingdom of Loathing: Some accessory items, mostly ones with especially powerful effects along with some for narrative reasons (e.g. it would be weird to wear two pairs of the same kind of shoes at once) can only have one unit of them equipped at the time. The wiki lists all of them here.
Multiplayer Online Battle Arena
- League of Legends:
- Some items have a "Unique" passive or active. When a player buys two items with the same Unique passive, only one of these passives takes effect. Additionally, in the case of Unique actives, after using one item both items go on Cooldown.
- The Stopwatch item exists solely for its active ability, which is to self-inflict a "stasis" effect that renders its owner completely immobile, but also completely immune to everything for a few seconds, allowing them to potentially survive incoming fatal attacks. It only has one use before it becomes a "Broken Stopwatch" for the rest of the game, and considering how the item goes for a not-insignificant price and otherwise provides no other stats, it should be used carefully if they don't intend on buying the upgraded Zhonya's Hourglass (the ability recharges after a long cooldown, but the item is mostly for AP mages).
- In Mega Man X3, you have the ability to swap out with Zero. It can only be used once per stage and if he dies, you lose him for the rest of the game. He's also needed to get his saber near the end of the game.
- Super Mario Bros.:
- New Super Mario Bros. Wii: Downplayed. The game has four playable characters. If you play the multiplayer mode, each player must select a different one. However, the only non-cosmetic difference is that each character has their own stock of Video-Game Lives, which is saved between sessions.
- Super Mario 3D World has four selectable, mechanically different playable characters. If you play the multiplayer mode, each player must select a different character.
- Gems of War has One-Shot spells, which can only be used once per battle.
- Mario Kart Wii: Downplayed. The local multiplayer mode disallows two players from picking the same character. This is mostly for flavour reasons, but the characters do provide a few character-specific stat boosts.
- Your pets aren't capable of reading the same book twice.
- In the Battledome, each pet can only equip one item that can heal, and one item that can freeze the opponent. Other equipment may also have similar restrictions on a case-by-case basis. Some items that can potentially heal and freeze, such as the Sword of Lameness, will occupy both slots.
- Battlezone II: Combat Commander only allows one copy of the factory, and a few other buildings (such as the bomber bay). If they get knocked down, they need to be rebuilt.
- Command & Conquer series:
- In the original game, the player's Temple of Nod only had the capability to launch a single nuke (it was a lot more powerful than nukes in the later C&C games though). GDI's counterpart, the Ion Cannon had no such limitations, and the AI player in the campaign could launch one whenever the player reached given checkpoints on the map.
- Command & Conquer: Generals: Unlike other games in the series, there's no upper limit to how many superweapons a player can build, with cost and space being the only limiting factor. However, with the Zero Hour Expansion Pack, superweapons can be limited to only one building.
- In the Yuri's Revenge expansion of Red Alert 2, partway through the fourth Allied mission in Egypt you get one free use of the Psychic Dominator after you rescue Einstein from Yuri's clutches — he managed to sabotage it under Yuri's nose before your forces rescue him. He states it is unstable, though, and will self-destruct before you get a second use.
- Command & Conquer: Red Alert 3: Commando units and superweapons are limited to one per player at a time, but one Challenge mission lets you build as many commandoes as you want (justified as both sides using a cloning facility), while another sets you up against multiple Proton Colliders.
- One Game Mod for Command & Conquer: Red Alert 3 doesn't prevent you from building multiple Shinobi, instead it makes them all weaker the more there are (invoking Conservation of Ninjutsu).
- The call-in supertanks in Company of Heroes for the Axis factions, the King Tiger and the Jagdpanther. You get one per game, and God help you if Allied AT guns knock them out (and you don't have a Bergetiger to revive them).
- Dawn of War:
- In the original game and the WinterAssault expansion, there were very different squad limits, leading to endgame armies consisting solely of top-tier units (even relic units). Later expansions greatly restricted this both by lowering the units' cap to 1-3 and increasing their food cap cost (such as the Leman Russ tank going from costing 3 cap without a limit to costing 5 cap and limited to 2 at a time). Much complaining ensued among the fanbase.
- Commanders and Worker Units are limited to either one at a time or in small numbers, but cost no food cap.
- Empire Earth 2: Wonders are not just one per player, they're one per game: as soon as one player completes their wonder, it's unavailable to everyone else (even if they were building their own) and can't be rebuilt if destroyed.
- Homeworld Cataclysm: The huge cannon you weld to your mining-vessel-turned-mothership has the problem of only working once before requiring major repair. The ship it's attached to is initially civilian, and energy conductors and heatsink systems suitable to power a cannon about a third of its entire size were not included in the original designs. Searching for them is a major plot point taking up several missions — though it does, eventually, give you a cannon whose reload rate doesn't include an inter-mission cinematic.
- Starcraft II: In skirmish games, Protoss players may only summon and control one Mothership at a time until that one Mothership gets destroyed.
- Warcraft III: In skirmish games, you can have up to three heroes, and only get one of any type including the mercenary ones.
- Bug Fables: In the "Spy Cards" mini-game, your deck needs to have two mini-boss cards, but they can't both be the same one. This is as opposed to normal enemy cards, which don't limit duplicates.
- Darkest Dungeon: The game prevents you from putting more than one Flagellant in your party at a time. The gameplay reason for this is to prevent multiple Flagellants from using Suffer on each other to pass Bleed and Blight between each other in a loop to prevent them from ever dealing damage. When trying to put another Flagellant in your party, he says "the burden of suffering cannot be shared".
- Darkest Dungeon 2 limits your party to one of each class.
- The Elder Scrolls: Some games in the series feature Skill Books: particular tomes that, if read, automatically increase a particular skill by one level. However, the level boost only occurs the first time you read the book; even if you discover another copy of the same title somewhere else in the game, studying it won't have any effect.
- Granblue Fantasy: Quite a few characters have skills that can only be used once per battle. Most notably, each one of the ten Eternal characters has an incredibly powerful skill with an extremely long 10-turn cooldown before it can be used. In addition, some other characters have skills that grant them massive buffs but knock themselves out after a few turns, meaning anything you do with them until then will only have one chance to work in each battle.
- The Legend of Heroes: Trails of Cold Steel and follow-up The Legend of Heroes: Trails into Reverie have the Lost Arts, extremely powerful magic Arts that can only be equipped from powerful quartz that can only by completing a trial, typically defeating a powerful monster. These cost all of a character's EP to use and while EP can be restored (or you can use a 0 Arts turn to nullify the EP cost), you can only cast each Lost Art once per battle, after which it will be grayed out and the game will say that the Quartz has losts its shine. While many of the effects are useful enough to merit equipping the quartz despite these restrictions, simply equipping these quartz also grants powerful stat boosts.
- Miracle Warriors Seal Of The Dark Lord: The white orb is a powerful item that can cause devastating damage to an opponent when used in battle. It can be won by defeating a powerful monster or granted by a White Monk roving creature and is needed to defeat the Dark Lord. While not so unique as to be lost forever if used, you can only carry one at a time, so no chucking 'em like water balloons at the monsters.
- While the story mode generally doesn't care if you put multiple Pokémon of the same species on your team, official tournament rules, most Smogon rulesets and several in-game battle facilities prohibit duplicates. Official tournaments also tend to limit you to one copy of each held item. This is intended to encourage diversity in teambuilding.
- Only one Kyurem can be fused with Reshiram or Zekrom at a time. If you want to fuse another Kyurem with one of the two, you will have to undo the existing fusion.
- The player cannot have more than one Calyrex in either of their riding forms. If you have a Shadow-Rider Calyrex, for example, you cannot fuse a different Calyrex with a Glastrier or Spectrier until you unfuse them.
- Mega Evolution, Z-Moves, Dynamax, and Terastalizing can only be used once per battle, forcing the player to time it correctly and make it count.
- Technical Machines (TMs) were initially one-use items and often limited to one per game, so you only had one opportunity to teach Blizzard to a Pokémon. However, later games have done away with unique TMs by either making them reusable or letting every single-use TM be re-obtainable.
- Some moves, ranging from support (such as the stat-lowering Memento and the healing Lunar Dance) to attacks (such as the high-power Explosion or the damage-equal-to-lost-HP Final Gambit), make the user faint. As only items like Revives can bring back a fainted Pokémon, these moves can only be used once per 'mon under any ruleset that forbids items. The sole exception is the move Revival Blessing in Pokémon Scarlet and Violet, which is itself a single-use move that can only ever have 1 PP (though you can get a second use with a Leppa Berry or by bringing both users of the move).
- Fake Out and its stronger Bug-type equivalent First Impression are high-priority attacks that make the target flinch 100% of the time, but it only works on the first turn the user is sent out.
- There are moves that remove a user's typing on use, such as Burn Up and Double Shock, and once the user loses their typing they can no longer use that move again until they switch out or the battle ends. However, in Scarlet and Violet, Terastalizing can be used to get around this since Tera Types can't be removed, enabling multiple uses of these moves in a row.
Survival Sandbox Games
- The Dragon Egg in Minecraft generates only once per world in Java Edition, when the Ender Dragon is defeated for the first time. In Bedrock Edition, the Dragon Egg drops when the dragon is killed on the first and second times, averting this trope.
- Cities: Skylines: Unique buildings have special requirements for unlocking and provide popular landmarks for your city. As the name suggests, each unique building may only be placed once per map.
- Mega Man:
- In Mega Man Battle Network, Giga Chips are extremely powerful chips with devastating effects such as dealing heavy damage across a wide area or buffing Mega Man's stats immensely. But their power comes with the caveat that only one Giga Chip may be put in a folder at a given time, preventing players from simply filling their folders with them.
- The sequel series, Mega Man Star Force, has the same restriction with its Giga Cards, which are similarly powerful and thus similarly restricted to prevent abuse.
- In Warframe, an item can't have multiple copies of the same mod attached to it at the same time. Some mods (most commonly, a basic mod and its Primed variant) are technically separate but considered equivalent due to having similar effects, and thus can't be equipped simultaneously.
- Wonders have always had some restriction. Initially all wonders were required to be unique on the entire map, while later games relaxed it for ordinary wonders to just having one per player. And no, no refunds if someone else was building the same wonder as you.
- The console-only game (later ported to mobiles) Civilization Revolution only has one nuke in any single game. It's given to the player who builds the Manhattan Project wonder. It can hit any city or square on the map but leaves no fallout.
- X-COM series:
- XCOM: Enemy Unknown
- Rockets. Since the Heavy Class is carrying a light machine gun (the heaviest weapon in the game), he is already very burdened, so he can only carry one rocket, with no way to resupply during a battle. Well, technically, if he takes the right promotion perks, he can use two rockets and a special shredder rocket (which causes less damage but inflict a Damage-Increasing Debuff), still, with a maximum of three rockets, make them count.
- All active items work similarly. The medikit gets one use (three with a Field Medic Support), four when he gets Deep Pocket in Enemy Within), grenades can only be used once (twice with the Grenadier Perk from the Heavy, or the Deep Pocket support ability in Enemy Within), the Arc Thrower can only be used twice (thrice with the Support ability Deep Pockets), the support Smoke Grenade can only be used once, twice with the Smoke And Mirrors ability (thrice in Enemy Within). It had to be noted it applies only during missions, as the squad gets resupplied for free between missions.
- In the expansion XCOM: Enemy Within, there is a scripted event involving an alien attack on your base (if you lose this mission, it's Non-Standard Game Over for you). The means the Mind Control the aliens use on some of your techs to get them to sabotage your base defenses is stated to be a one-off.
- XCOM: Enemy Unknown
- Batman: Arkham City: The boss fight against Mr. Freeze involves using various objects to lure him into traps. However, as Batman states ahead of time, Mr. Freeze is smart enough not to fall for the same trick twice.
- In Pokémon: The Series, battle gimmicks are limited to once per battle much like their appearances in the video games. In addition, in the World Coronation Series arc of the Pokémon Journeys: The Series, all competitors are limited to just one gimmick and only once per battle so Trainers who don't have Mega Evolution, Z-Moves or Dynamax can still compete on fair terms with those that do. This rule is only lifted for the final fight of the Masters Eight Tournament, where Leon asks for an exception so that Ash can use all three against him. Eternatus also appears during the final battle to give both competitors a second Dynamax.
- Calvin and Hobbes: One of the few stated rules of Calvinball is that you can't play it the same way twice.