There is a realm some would call the in between space where souls stay after their body passes in a tragic way or suddenly and they have unfinished business here on earth. These souls can be earthbound and come out of this realm and into a body that just experienced near death or has died but revived. I will explain, let's say you have two people who die at the same time, however, one of those two people has left their physical body and trapped in what I call the In between space, that soul can enter another person's body. This is often referred to as a walk in. The other person's soul is literally pushed out of their body and roam the earth in the in between space. The other soul that walked in or stepped in is now in full control of the body. This is something that does not frequently occur but there are some instances where one soul is stronger and will fight to stay. Remember souls are energy and need a human body as a vessel to exist.
When a soul walks in they carry their memories, moods, gestures, and personality. This can cause others to think that this person has amnesia or they are posessed by a demon but in reality this is a human soul posession. The longer that the other soul who originally belongs to the body is out of the body they get weaker. This can become a war and both souls are fighting for room in the body. This person may exhibit signs of weakness, passing out, develop fevers, etc. Again I want to say this is not a common scenario. Those who also astral project out of their body and lose their way run their risk of staying in the between space. Trying to find a way back to their body. If another human soul that recently passes is lingering in the in between space, it can muster the energy to walk in. Some do believe once a walk in occurs that the old soul is no longer in the body or can come back in. There are other cases where it is also believed that the soul is another soul but retains all of the memories etc from the previous person that inhabited the body. What is certain here, is that some souls will fight to finish their business here on earth.
A person who has experienced a walk in do come back changed and often remember seeing the light, they might even feel confusion. Some cultures like native americans or shamans can perform certain rituals to get the soul that belongs in their body back. Some walk ins don't even need that. They have free will and will eventually go back into their body permanently.
Family members or loved ones will know when a walk in happens because that person may no longer act or even appear to be the same. They may have knowledge how to do things that the other had never done before like knowing how to play a musical instrument, knowing how to swim or no longer fearing things that the other person feared. Phobias or blocks may just disappear.
It is important to make sure you dont have any unfinished business, to get grudges out of the way, to live as much as you can in a state of gratefulness and love. Appreciate everything around you. If you astral project a lot, you can use crystals like hematite to ground you. Meditating will also be good.
Souls will always be around us, and it may not be necessary for a spirit to walk into another body because through the use of divination or mediums, messages can be channeled. Spirit can personally communicate through dreams. Stay aware and awakened.