Kilgore Trout
Misanthropic Humanist
I'd guess it was more common, but only based on anecdotal evidence.
I've found them to be extremely common, and in fact, "multiple orgasms" tend to be clusters of g-spot/vaginal orgams rather than clitoral orgasms. I find that women who masturbate frequently, or are comfortable with their own bodies/sex, have an easier time having both types of orgasms, but many women are less familiar with vaginal orgasms unless they have experienced them via a partner already.