A sensation that this has happened before is really a glitch actually, and it demonstrates that something has quite recently been changed. Somebody or something has stopped to exist, all recollections and records of their reality eradicated until the end of time. A sensation that this has happened before happens when they get into your cerebrum, when they have to change your recollections. Perhaps to eradicate your sibling from the world. You know, the sibling that you never had.