Resonants

Not to be confused with resonant, an alternative term to introjectundefined:

Resonants are headmates that seem to be made "of the same stuff" and resonate with each other, to the point where had one not formed/arrived in the system before the other, they would have been the same person. They may also be versions of each other from alternate realities, or one may be a reincarnation of the other's past life.

Related Terms
Fenitive, seelen, and versiomate all describe headmates somewhat similarly connected to each other.