This is a common scenario, often referred to as dual personality or dual identity, and it's common in many novels. It had been a long time since the main character had another person who was exactly the same as him but had a very different personality. This kind of situation usually appeared in novels or anime where the protagonist had a personality that was very similar to himself, but there was a huge difference between the two. This personality would usually appear at night and do something opposite to the protagonist's original personality to push the story forward. It should be noted that this situation is not common in real life. If it appears in real life, it may need to be explored through scientific means.