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Key Finding 80 - A spirit cannot exist simultaneously in heaven and in a body on earth, or in two bodies on earth. However, when a spirit reincarnates, a residual energy is left in heaven that is able to communicate but this is not really the spirit itself that is there.

Afterlife 101 Section, Chapter 3

Events when individuals believe they are going to "hell"
Spirit Guide: Those who had a vision that they were going to go "to hell" have many different versions of what hell could mean to them. There will be those who have a typical vision of what hell is, and that is what they will experience. There are those who will think hell was certain events or circumstances in their life and that is what they will experience for a period of time. This is not something that lasts for any great deal of time because spirit will come to those and those experiences and give them the choice to see that they can move out of those experiences and that is not what life after life is all about. That is not what they have to look into for infinity--they are able to move and experience their loved ones, see that they are there and around them and open into the light of love. It's as if butterflies were emerging--and that is somewhat what happens to souls with those concepts of heaven and hell and nothingness at the time of death.
Question: Are there ever cases where a spirit has been reincarnated before it has died in a previous life and arrived in heaven?
Spirit Guide: With our knowledge no. We don't have all of the knowledge but as far as we know, no, that is not the case. As on earth, no one energy can occupy two spaces at one time. And so a spirit, though there is a residual energy left in heaven to greet those who come to earth because that is what you are expecting, it is not really the spirit itself that is there. And we do not believe that an individual who has died can be in heaven and in a body at the same time. Is that what you are suggesting?
Question: Or in a second body while it's apparently still alive in its soon-to-die body.
Spirit Guide: We do not believe this is the case at all. As your spirit guides we see that generally speaking a soul does not immediately incarnate and especially not two at one time.

Key Finding 211

Spirits never live parallel lives on earth simultaneously.

Afterlife 101 Section, Chapter 6 - Parallel lives

Question: Do spirits ever live parallel lives on earth simultaneously?

Spirit Guide: No, they do not. Do you mean one spirit in two separate bodies?

Question: Yes.

Spirit Guide: No, that is not the case.

Agreements

Destiny of Souls by Newton

The ability of a soul to unite with itself is a natural process of energy regeneration after physical death.

The Spirits' Book by Allan Kardec

Question: Do spirits have the gift of omnipresence?  In other words, can a spirit divide itself or exist at several points of space at the same time?
Spirit:  No. A spirit cannot experience self-division.

The Spirits' Book by Allan Kardec

Question: Can the same spirit incarnate itself in two different bodies at the same time? 

Spirit: No, the spirit is indivisible and cannot animate two different bodies simultaneously.

Disagreements

Destiny of Souls by Newton

...all souls who come to Earth leave a part of their energy behind in the spirit world, even those living parallel lives in more than one body...if someone you loved died thirty years ahead of you and has since reincarnated, you can still see them again upon your own return to the spirit world.

Destiny of Souls by Newton

The energy of the soul is able to divide into identical parts, similar to a hologram. It may live parallel lives in other bodies although this is much less common than we read about. However, because of the dual capability of all souls, part of our light energy always remains behind in the spirit world.

Journey of Souls by Newton

Most of my colleagues who work with past life clients have listened to overlapping time chronologies from people living on Earth in two places at once...What we must appreciate is, if our souls are all part of one great oversoul energy force which divides, or extends itself to create our souls, then why shouldn't the offspring of this intelligent soul energy have the same capacity to detach and then recombine?

Journey of Souls by Newton

Those souls who are currently incarnated in one or more bodies at the moment may not be actively engaged with welcoming anybody back...radiate a dim light with low pulsating energy patterns and don't seem to communicate much with anyone.

Destiny of Souls by Newton

Many people feel it is common for souls to live parallel lives. I have found this not to be true at all. The souls who choose to split into two or more bodies within the same general time frame on Earth want to accelerate their learning.

The Wisdom of the Soul by Ian Lawton

Can a soul have two or more incarnations at the same time?....they were unanimous that it is indeed possible....They were also unanimous that this is usually done in an attempt to accelerate the learning process by working on two life issues at once....Most of our sources who were asked how commonly this option is used seemed to suggest that it was rare.

Seth Speaks by Jane Roberts

Question: You said that some personalities can be a part of more than one entity? 

Seth: There are not boundaries to the self, and no barriers put upon its development.  A personality may "originally" be part of a given entity, and on its own develop interests quite different.  It can on its own take a lonely way, or it can instead attach itself or gravitate toward other entities with interests like its own.  The original connection will not be severed, but new ones made and formed.

{Prospective Questions}

Question: 80, 211

Are there any circumstances where a spirit is able to have two or more incarnated lives on earth simultaneously?  If so, what would be the adverse results, if any?