Dissociative Identity Disorder or Dual personality is a rare but common mental illness among young people. It is a state whereby individuals who have gone through traumatic experiences at a young age develop different personalities different from their original selves. This personality tends to have its name, characters, behaviors, thoughts, and feelings. They are often cold, cheerful, or mostly violent in contrast to their original personality.
I once had a friend who I never knew had Dissociative Identity Disorder till the day he acted like a different person altogether. I was confused and curious about what happened. I had to do research and when I realized he had a personality disorder, I knew I had to keep a distance from him to avoid getting hurt till I knew what triggered him because I couldn't tell who was in control or which personality was making the decisions. Probably one of his personalities likes me and the other dislikes me so for my safety I had to distance myself from him.
But in the course of all this, one thing I have come to learn and would advise anyone who knows a person who suffers from this condition, be it a family member or a Friend, avoid topics, activities, or events that trigger them to switch to another personality, make them feel loved and understand their situation, support them unconditionally and help them seek therapy to recover from their traumas. This will help greatly individuals with personality disorders.