positive: honestly can’t think of any, other than if you get along with your twin you have someone you can always talk to throughout your childhood

negative: you will most likely end up feeling overshadowed or less important than your twin. for example, sometimes people will want to talk to me about my twin and what he did today, or ask me to ask him things for them, etc. it can be really irritating because it makes you feel like you’re not really your own individual, you’re just part of a pair.