Double allows Jin to create a copy of anything, two at a time, through touch.
With Double, Jin can produce replicas of anyone to provide support in numbers. In order to conceive a perfect clone, Jin requires a knowledge of precise measurements and physical features. Clones possess the same personality and abilities as the original, making it hard to distinguish them from the real person at a first glance. The copies only recall the memories of the last time they interacted with Jin himself.
Jin has a limit of creating only two clones at a time. However, because Jin is able to copy himself and his copies also have access to Double, he is theoretically capable of producing infinite replicas of himself by letting his clones duplicate themselves further. This gives Jin the manpower to realize a large task easily or several smaller ones at once, provided that his clones cooperate with him. Tomoyasu Chikazoku claims that Double can even replicate dead people.
While Jin himself doesn't suffer any repercussions from his clones getting destroyed, he is still capable of detecting their destruction, even if they're far away from his location.
As already said, to create a clone Twice needs a flawless image of whatever he plans to duplicate stored within his mind, so the measurements and physical features has to be extremely accurate. Without a perfect image, all he will get is a poorly formed copy that won't last long. Therefore, every time a person undergoes any physical change, Twice has to take the measurements again.
The copies are less durable and powerful than the real thing, dissolving into a viscous liquid after receiving enough damage, and a second copy is apparently even more fragile (according to Jin, physical injury equivalent to a broken bone is usually enough to extinguish a clone). And every clone created by every Jin clone is even more fragile.
Double had a major role in shaping Jin's character, to the point of making him insane. In the past, Jin initially used Double in order to cope with loneliness, since he had no friends or relatives to interact with, but he eventually devolved into criminal life, committing theft and robberies alongside copies of himself.
Jin would abuse his copies to act as servants and follow his bidding. However, due to Jin's clones sharing the same personality as the original Jin (who was lazy and overbearing), they disliked the slave work they were put in and ended up rebelling against their creator, wrapping him with ropes. As each clone felt they were the real Jin and the other were fakes, they started fighting amongst themselves while the original Jin could only watch in horror as his own image kept attacking and killing itself repeatedly.
Having suffered a traumatic experience with this particular event involving his Quirk, Jin became afraid of using his ability on himself again. If he is left through long periods of time without covering his face with a mask, Jin, as he claims, will end up splitting himself, although it's unknown if he is referring to an actual physical consequence or just a metaphor for his multiple identity disorder. During the Meta Liberation Army Arc, due to a strong desire to protect his comrade, and receiving injuries, which confirmed that he is, in fact, the original and not just a clone, Jin overcame his trauma, becoming willing to duplicate himself once more.
Having no qualms in replicating others, Jin is a very useful asset to the League of Villains as his Quirk grants him a way to temporarily increase their numbers if they find themselves short-handed, giving the organization more "subordinates" to attack or distract enemies. Giran claims that Jin could potentially take over the entire country with his ability if used correctly. Because of this, Tomura Shigaraki considers Jin a vital part of his villainous goals, and the Meta Liberation Army have shown interest in drafting Jin into their own ranks.
So far, Jin has copied the following people:
- Infinite Doubles: Sad Man's Parade (無限増殖
哀れな行進Mugen Zōshoku Saddo Manzu Parēdo?): The user creates clones of themselves, then the clones create clones of themselves, then those clones create clones of themselves. This cycle continues, resulting in thousands of clones of the user.
- This Quirk is functionally similar to Clones, as they both revolve around creating duplicates out of a viscous liquid. There are a few key differences, though:
- Ectoplasm can only make replicas of himself, whereas Jin is capable of making doubles of anyone, including himself as well.
- Ectoplasm can create up to 30 (36 at best) clones, whereas Jin is limited to only 2 clones of other people. It is worth noting that Jin can create as many clones of himself as he wants, as those clones can use their own Double Quirk on themselves to continuously multiply while bypassing the 2-clone limit.
- Ectoplasm's clones can't use his Quirk, whereas Jin's clones can.
- Ectoplasm can create a massive clone of himself, whereas Jin has not shown such an ability.
- Despite creating a copy of Himiko Toga in the Shie Hassaikai Arc, Jin later claims in the Meta Liberation Army Arc that Himiko never allowed him to take measurements of her.
- This is a mistake on Kohei Horikoshi's part, which he acknowledges on Twitter, and says he'll change it before the volume containing said chapter comes out.
- Cloning - Wikipedia article about the process Jin's Quirk was based after.