Just a curious random thought: can a person be haunted AND possessed?
Not that the ghost in your haunted house possesses you; but you are possessed by a demon and - in additional to that - taunted / harassed by a separate & secondary malevolent spirit or ghost while being possessed?
Would the demon retaliate against the ghost? Would he think that was terribly funny and ironic? Can demons and ghosts "double dip" on the same person?
(wonderful stuff this vodka and coffee 
)