One-Curse Limit

When a world's rules state you can't curse someone who's already cursed.

This trope centers on situations where an attempt to cause Standard Status Effects fails because the intended victim is already being affected by something. It could be the same effect from a different source or else the fact that they were hit by something separate previously is preventing a new effect from working.

Not to be confused with Acquired Poison Immunity, in which a character is resistant to a poison or effect because they'd been affected by it previously and recovered. This trope is when an attempt at poison outright fails because they're currently affected by something else.

This sometimes overlaps with Hybrid Overkill Avoidance if the Curse would turn the character into a monster. For example, vampires can't be cursed with lycanthropy and vice versa. See also Disability Immunity, which this can be a sub- or Sister Trope to.

Not to be confused with only allowing one swear word in a work.

Anime & Manga

 * In the Berserk universe, a person who's already been branded for sacrifice by the Godhand on behalf of another cannot be made a sacrifice by anyone else. Since most people who get branded do not survive their initial sacrifice, cases of branded survivors are vanishingly rare so this seldom comes up, but during the third major manga arc (the last one before the Golden Age arc), the Count tries this with Guts only to be told by the Godhand that he can't sacrifice him because of what Guts already went through during the Eclipse, and because Guts is an enemy, not someone the Count loves.
 * In the Death Note universe, once your name is written in a Death Note, you can't be affected by any other Death Note. Of course, since you're going to die within the next 23 days at most, it's rather cold comfort (but see the movie, below).
 * In Dragon Pink a magic sword obtained after defeating a monster had a death curse on it: Santa immediately falls sick and his lifeforce is slowly being drained away, but Pink (who's already cursed) is not affected.
 * In One Piece eating a second Devil's Fruit is supposedly fatal.

Film

 * In the movie version of Death Note,, because he had already "cursed" himself with another one.

Literature

 * In Princeps' Fury, book five of the Codex Alera, Amara is able to resist the effects of a Discipline Collar, a Slave Collar that causes ecstasy in the wearer, by having one put on her by someone else prior to getting captured.
 * Inverted in Patricia Wrede's short story "The Sixty-Two Curses of Caliph Arenschadd".
 * A form of this occurs in the Discworld book Making Money when Topsy Lavish has a contract put out on Moist von Lipwig. This actually protects Moist, as the Assassin's Guild won't take two separate contracts on the same person and Moist is only in danger if the dog doesn't live out a natural lifespan.
 * In Alcatraz versus the Scrivener's Bones, Attica Smedry foils a Deal with the Devil by

Tabletop Games

 * Averted in Munchkin, where the rules state that curses (or traps) may be played at any time. A player can get a pile of curses at once, but then, driving your opponents to tears is a viable strategy if it makes you win...
 * Dungeons and Dragons 1st Edition AD&D. Insane creatures were immune to psionic attack.

Video Games

 * In Dragon Quest VIII, is already under the effect of a curse, which is why the spell on the castle didn't effect him. In an aversion of Gameplay and Story Segregation, he cannot be placed under the Curse status effect in battle, either.
 * In Pokémon, any given Pokemon can only be under one of the major status effects (Sleep, Paralysis, Burn, Freeze, Poison) at a time. They can still be affected by secondary status effects like Confusion, but a burnt Pokemon can't be paralyzed, and a poisoned Pokemon can't be frozen.
 * In fact, in high level Pokemon players it consider normal to allow ones own Pokemon to be inflicted with one status that isn't much of a bother (or with some special abilities, a benefit) purely to prevent more crippling ones.
 * In the Shin Megami Tensei games(well, at least the Persona games, I don't remember the other ones) you can only get hit with one status ailment at a time. As long as you have one, any other ailment will miss.
 * In Final Fantasy games, being zombified grants immunity to poison and petrification. Regeneration deals damage though.
 * One boss in Bunny Must Die attempts to curse Bunny in his death throes. It doesn't work, because she was cursed in the backstory.
 * This can be exploited in Seiken Densetsu 3 to heal all status ailments on the party for free..
 * Warlocks in World of Warcraft work this way, in that a Warlock can only apply one curse-type spell, such as Curse of Weakness or Curse of Tongues, on a target at a time, with any new curses overriding the previous. In fact, two strong spells that were once curses had to be made into a new type (Banes) so that Warlocks could use them with their other curses. Though it is played with since a target can have multiple curses at a time so long as each curse was placed by a different Warlock.
 * A spell specific to the Affliction specialization, Unstable Affliction, has this relationship with its counterpart in the other two specs, Immolate.
 * In Elder Scrolls, a werewolf cannot become a vampire, or vice versa. Of course, seeing as the Dragonborn or any hireling he has can become either, being both would make the player ridiculously overpowered.

Web Original

 * In the second season of the Random Verse, Joker's attempt to turn all the heroes into brainwashed Darker and Edgier versions of themselves doesn't work on Spider-man because Mephisto has already altered his mind.

Western Animation

 * The Simpsons Treehouse of Terror: The Devil demands Homer's soul, but he earlier promised his soul to his wife Marge, so the devil can't collect.