It isn't unheard of. Its not possession. The most common spiritual attack is oppression. Now this link explains it better than I can.
https://www.gotquestions.org/demonic-oppression.html
However you got to make sure its not schizophrenia and/or other psychoses.
IF you think you're dealing with demonic oppression, then you'll need help from the opposite side of the spectrum. While you're studying how to come to term with this, I found out these demonic influences don't manifest if I play Christian praise music.
I knew I was being oppressed in my late 20's when it kept inserting thoughts in my head, which I knew didn't come from me. It was so convincing because it was able to imitate my inner mind voice. You know how we talk in our head to ourselves (like studying quietly etc) Well it did that. The reason I became vulnerable was I opened up myself for spiritual attack because of highly mistaken psychic experiments during my teen years. Its a form of spiritism also.played spirit of the glass too. I didn't realize I actually gave permission for these dark spirits to waltz into my life. I was being oppressed heavily but when I heard this Christian radio station playing Praise music,,it just stopped and I had relief I haven't had for months. To make a long story short, it was a journey that took me out of the dark and back to having my real self again. I'm not saying this is what you're going thru OK? I just want to share what worked for me.