I've had many a mare here scanned and found twins at the first ultrasound (usually within the 15-30 mark), only to have one "disappear" by the second scan at the 3 month mark. Usually, nature takes care of this problem for the little ones. I did have a friend who had twins carried to birth, but usually, one twin is very weak, and in her case, only 1 survived after the 1st day.
Only had 1 mare who had to have a twin "pinched", as she was carrying it at the second ultrasound. So my vet took care of the problem, as she was a small mare, and never would have carried twins to term.