Multiple Personality Disorder
1.) A disorder characterized by the presence of two or more distinct personality states
2.) between 20,000 and 200,000 thousand cases are recorded per year.
3.)It can actually be hard sometimes to tell if you have Multiple Personality Disorder because only one personality knows they have another personality. Generally people around you will see you acting strange.
4.) The person should see a psychiatrist and the psychiatrist will prescribe medication and see what works for that person.
5.) They can go to a psychiatrist.
6.) Their doctor can talk to them about it and walk them through it. Usually it is stemmed from something deeper and some kind of trauma and they need to figure out what that is for them.
Sources: Wikipedia.com and I took the picture