Does the complete soul return to the Oversoul? Or will fragments stay behind?
A human incarnation only utilizes a small part of the Oversoul's energy. Most of it stays in the Spiritual reality, and also can be incarnated on other planets or worlds and create parallel lives on Earth. You do not die in one life and reincarnate with exactly the same energy. The energy returning from a human life rejoins the Oversoul, its knowledge and learned lessons are integrated into the information banks of the Oversoul, and a new offshoot is created for the next life.
The astral body and personality of the past life is usually discarded, though it might be stored somewhere like a used carnival costume to be recreated if necessary. The only time when a soul reincarnates in exactly the same energy and astral body is when it does not cross over into the Spiritual Reality after death, it can either stay as a ghost or attach to somebody, and in some rare cases the soul immediately reincarnates into an available fetus without any plan or contract with the souls of parents, and in such a case the entering soul did not get permission to join the parent group and are often disenchanted by certain family members.
These are cases of children who vividly remember their past lives, they died traumatic deaths in those lives and reincarnated instantly without any healing or life review, that's why they usually have a lot of phobias and vivid memories of their last life and death, as they were not veiled (given amnesia, or cross and bathe in the River of Forgetful) on the way from Spirit World back to 3D Earth. Souls of babies or small children also stay around their mothers' energy fields--if they died young or were stillborn--and reincarnate in the next pregnancy (it is more complicated with abortions or older kids and teens).
When this happens, returning back to 3D human realm without prior planning, the newly incarnate will face a lack of resources and opportunities as nothing has been aligned and agreed to, usually souls do this for a quick return for revenge purposes.
When a part of the astral body stays on the lower level of the Afterlife while another part reincarnates--in the case of Split Soul Twin Flames separated from one soul due to severe trauma. In this case a part of astral body is put into hibernation in Places of Rest in 5D, and another, healthier part is quickly cleaned up, it rejoins the Oversoul and the Soul Group, and continues to reincarnate. But they eventually will have to meet again in human lives and unite. It does not matter if the soul reincarnates within a year or longer.
Tags: soul, spirit, oversoul, split soul, twin flames, reincarnation