I posted here on the Afterlife Knowledge Forum a couple weeks ago :
http://www.afterlife-knowledge.com/cgi-bin/yabb/YaBB.cgi?board=afterlife_knowledge;action=display;num=1138917286In cases of *identical* twins, because the physical body & brain is genetically identical (after fertilization by a single sperm with a single ovum, the resultant zygote for various biochemical reasons split into two, both of which develop into viable foetuses), and thus being vibrationally similar for the occupying soul, it is a possible option (one that is sometimes, not always, exercised), for the same human soul to occupy both bodies at once.
Thus, in such a case, the two individuals would have the same soul or higher self. Some souls might choose this (prerequiste being, of course, there are suitable upcoming identical twin biological bodies, whose upcoming life/lives present an appropriate karmic option for the soul in question, in terms of karmic connection with family members, compatibility with the soul's existential program or planned objectives for the lifetime), because to the soul, it means greater opportunity to accomplish learning and work within the same amount of time, akin to multi-tasking.
Extremely rarely (though theoretically possible, but not practically advisable), does a single human soul incarnate simultaneously into two or more physical bodies, because of the increased difficulty or challenging nature, for the higher self or soul to simultaneously operate or function as two individuals at once (note that we're referring to the level of the individual soul, not the oversoul or disk/disc level). But in the case of genetically identical twins, this becomes less strenous for the soul to do so, and thus, it becomes more feasible of an option.
Obvious advantages to simultaneously incarnating as identical twins, would be gaining more experience, furthering more relationships, getting more karmic work done, within the same lifetime span.
Note that this is only an option, which is in practice, rarely exercised. Certainly not all genetically identical twins have the same soul or higher self. But in cases in which genetically identical twins might have curious synchronicities, for instance both experience events with great personal or emotional significance at around the same time, or discarnate/depart from the physical at round the same time, and other such coincidences that goes beyond normal telepathy or psychic attunement.
In such cases which might seem otherwise enigmatic to the observer, understanding the possibility or mechanism of identical twins having the same soul or higher self, will allow for greater intuitive understanding on the curious synchronicities.