Naruto is an animation that officially begins to convey the traditional culture of Japanese ninja. Anime "Huo Ying" is believed to be familiar with various body double techniques. Speaking of body pairs, I believe that the most common "wooden" body pairs may come to mind. However, that is only the lowest level of ninja.

For example, the specific principle of Uchiha Shisui's teleport is ominous, but according to the author's description, each doppelganger is neither a ghosting nor a shadow doppelganger. It should appear in several spaces after it is extremely fast. As far as I know, this instant technique is extremely strong and has not been cracked.

Then there is the shadow doppelganger that Naruto often uses, and the shadow doppelganger is listed as forbidden in the play. It is forbidden because every split shadow member has independent thoughts and consciousness, and memory is not affected by space. This kind of two places at once with independent thoughts can't be cracked by conventional ability!

The last kind of Ninjutsu is a technique of reincarnation from dirty soil created by the second generation of Huo Ying. Its principle is simple, calling the souls of the dead to fight. The real horror of metempsychosis is that it has no side effects. Even if the performer dies, this technology will not be solved unless the performer solves it himself. So I think metempsychosis should also be unsolvable.