Twins are just fascinating. I imagine that people who aren't twins really can't comprehend what it's like to have a twin sibling. Perhaps people with a sibling differing only nine months in age have a vague idea, but I bet it's really pretty surreal, and everyone without a twin will never really "get it."
I haven't known too many twins in my life. Just about every lunchtime during high school I hung out with two twin sisters. They were certainly a strange pair. They were identical twins; after all that time eating with them I still couldn't tell them apart. They were both reasonably smart, both were interested in music and the theatre, and they never left each other's sight. I also know a pair of twins both working as engineers at Microsoft, though at least I can tell them apart.
That's just got to be so strange. While you can generally tell that I'm related to my brother, we certainly have wildly different interests, and we're three years apart, and there have been many points in our lives where we wanted nothing to do with each other. He's very extroverted, and I'm very introverted. He's buff, and I'm tubby. He likes to shoot things, and I like to code things. I can't quite imagine what it would be like if we were the same age, looked alike, and had mostly the same interests.
Would I be able to stand spending significant amounts of time with another me? I think that's an interesting question for anyone to ask themselves. I think I could... probably... most of the time. But I don't want to kid myself into thinking that I really have any clue what having a twin would be like.
I wonder if having a twin sort of automatically "fixes" personality flaws. If you and your twin both have the same annoying characteristic, it would seem that you'd notice it and constantly be reminded of it, and be much more motivated to correct the problem.