MULTIPLE PERSONALITY DISORDER is not demonic possession,
And possession is not MPD.

Many exorcists know about multiple personality disorder and disociative disorder:

Formerly known as multiple personality disorder (MPD), DID results in a person's enactment of two or more selves, sometimes called "alters." Persons with DID may have tens or hundreds of selves, more or less developed: some speak with distinct voices, and have different names, likes, and dislikes. The alters' drawings, handwriting, and accents may differ. Some of them identify themselves as demons, causing well-meaning ministers to try to cast them out.
Psychologists explain the controversial disorder in four ways, says John E. Kelley, director of Biola Counseling Center in La Mirada, California:

1) DID results from a severe trauma, which usually takes place in childhood and often surfaces through controversial "recovered" memories. DID leads to fragmentation into at least two selves (one of whom is often an abused child). That is why survivors of alleged ritual abuse are often diagnosed with DID.
2) DID is a role-playing phenomenon that may or may not be based in a real-life trauma. "Dissociatives" play different roles because they are affirmed for doing so.
3) DID is faked by people who want attention.
4) DID is born in therapy. The disorder is brought on by therapists who use suggestion (intentional and unintentional) through which they end up convincing their patients that they have dissociated identities.

But, even they know that, some exorcist, in some cases they mantein that there is possession. They know what a psychiatryst knows. It is not a fight between science and superstition.