A common quirk of a creature with multiple bodies, or 2/more creatures/person who are "linked" in a deep manner, or a Hive Mind group, is that all of them have to be defeated at nearly (if not exactly) the same time, otherwise the killed individual will regenerate/revive itself.
There are many explanation for this, including but not limited to: extreme regeneration ability that's apparently shared by two people, ability to duplicate themselves or breed a huge number of them in a short time (for Hive Mind cases), or the individuals being able to heal/resurrect their own dying comrade. Killing all of the entities at once would thus prevent any individuals to come back.
- 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.
- Similarly, the Twin Card in Cardcaptor Sakura needed to have both halves of it defeated at the same time, and Syaoran and Sakura found that attacking them separately was ineffective. Luckily Meiling had trained with Syaoran in martial arts and they were able to carry out a coordinated attack to defeat it.
- 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.
- Soul Eater: Troubled Souls: This is main reason why the Brother and Sister of the Gemini were such a difficult Kishin Egg to locate and kill. They are actually two entities which had to be destroyed in their combined form or else they would keep coming back. Maka and Caius found attacking them separately is a fruitless endeavor because the "killed" half would simply find the other, recombine, and then split again into two beings good as new. To make matters worse, the separate halves are big on working together to survive.
- Robot Wars entrant Gemini is the Trope Namer. If 50% of any competitor's weight was destroyed/rendered immobile, the competitor was counted out; for Gemini, which was a clusterbot that split into two smaller robots after entering arena, this required either completely destroying/immobilizing one of the two robots or destroying/immobilizing 50% of each robot. This rule was removed later.
- 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.
- In the Animorphs prequel book The Ellimist Chronicles, Toomin (the Ellimist back when he was mortal) is captured for years and years by Father, a massive tentacled sea monster which has absorbed the minds of everyone it's every killed. Eventually Toomin learns to fight back and absorb the minds into himself. He states that the last mind he absorbed was Father himself, which turned out to be nothing.
- Dungeons & 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...
- It's common practice to give a Dual Boss revival skills, such that if one of them is left alive they can revive the other, and force the player into trying to take them out at the same time.
- In Tsukihime, Nrvnqsr Chaos can only be killed if all 666 creatures that make up his body are killed at once. If they are killed individually, he can instantly regenerate them. This fact makes him nigh-impossible to kill, even for top-tier fighters like Arcueid. Shiki, however... when he kills something, it stays dead, no matter what. So he's able to kill pieces of Nrvnqsr without them regenerating, which freaks the monster out.
- Lufia II: Rise of the Sinistrals had a dungeon boss that split into two, one form being fire-based and the other ice-based, which had to be defeated simultaneously. When the party first fights it, they fight each body one at a time; it regenerates and laughs in their faces. So they split the party in half and two characters fight each body.
- 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.
- 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.
- Later duplicating alien Echo Echo doesn't have this problem. All of the Echo Echos we saw in one fight were wiped out by the enemy (namely a Not Himself Kevin) but Ben survived by keeping one of himself in the trunk of his car.
- If any population of a single species is reduced below a certain limit, that population will die out. Humans need at least 1,700 people to repopulate the earth.