Gemini Destruction Law

In most media, to kill a Hive Mind or cluster creature you will need to kill every last part. Other times you'll luck out and only need to kill the core (or Hive Queen). Then there's this. Basically, it turns out that the monster needs those parts, so if you kill enough of them it'll die.

Contrast From a Single Cell.

Anime and Manga

 * In Neon Genesis Evangelion, Israfel was actually two entities which had to be destroyed at exactly the same time or else they would keep coming back. Similarly, Iruel was a nanovirus that kept growing and adapting until they managed to trick it into an evolutionary path that led to self-destruction.

Comic Books

 * Jamie Madrox of X Factor can be temporarily incapacitated when a foe kills any of his duplicates in proximity to him. That would be within a few miles, rather than, say, across the planet or in another dimension.

Film

 * In the film adaptation of Hellboy, the Demon Sammael comes back to life in twice its numbers unless all of them are killed at once.

Live Action TV

 * Robot Wars entrant Gemini. If 50% of any competitor's drive was destroyed, the competitor was counted out. This rule was removed later.

Literature

 * His Dark Materials: In an alternate universe, humans have shape-shifting animals called "Daemons" that are basically the other side of their soul. Because of their metaphysical connection, most human-daemon pairs can't bear being separated for more than a few yards as it causes pain and severe mental and emotional trauma. If either the daemon or human dies, the other dies with it also.

Tabletop RPG

 * Dungeons and Dragons 1st Edition supplement "Fiend Folio": The Cifal (Colonial Insect-Formed Artificial Life) was a monster made up of a huge number of insects bound together into a humanoid form. It had 10 hit dice altogether, two of them being the binding energy that kept the insects together. If you did two hit dice of damage to it, the insects broke apart and fled. That is, if you did 20% of its total damage, it was defeated.
 * Course, then you've pissed off a whole hive of moderately intelligent bugs, who, being demonic, are probably going to want revenge...

Video Games

 * In Halo, the Flood always have a Hive Mind, but it will attain a form of sapience called a Gravemind if the Flood attains enough biomass. Thus, the Monitors on the Halo rings keep the Flood in their labs isolated from any life-forms it can use as a host. 2401-Penitent Tangent, in the second game, neglected his duties, allowing a Gravemind to form on his ring, while either the humans or the Covenant in the first one released the Flood from a lab they thought was a weapons cache.

Web Comics

 * This is referenced in the webcomic Freefall that Florence's species would need 50 individuals for short-term survival and 500 for long-term. Her creators only made 14 Bowman's Wolves total so far, hence one of her main objectives is to ensure they continue.

Western Animation

 * Ben 10: One of Ben's alien forms was Ditto, which had the ability to clone itself without limit. Unfortunately their link means that if one clone is hurt, the other clone feels the same pain. In "Divided We Stand", one Ditto was captured by Mad Scientist Dr. Animo and and powers taken to multiply his mutant army, which all died after one was defeated.

Real Life

 * If any population of a single species is removed enough, the species will die. Humans need at least 1,700 people to repopulate the earth.