Oo all of this is interesting.
I don't have twins but My younger sisters are ID twins. They've never had a DNA test but gosh they look alike. They are almost the same person besides a personality where one is quiet one is loud and one is slightly taller then the other by like 3 cms. But many people can't tell them apart besides me.
Family warns me I have a high chance to conceive twins but this my first viable pregnancy but I've only fallen with singletons. My mum was 25 when she fell with my sisters and it was 100% natural.
She told me they found out at 12 weeks one was hiding behind the other but she can't remember if they shared placenta or anything else helpful. So the rest is a mystery.