Honestly, it really depends on the University. Some universities do not allow you to do a masters in the same field, twice - for example : http://cs.stanford.edu/admissions/faq#a2

Some of them will waive GRE/GMAT scores if you hold a master's degree (couldnt' post the link here, apparently I need 10+ reputation to do this). Other universities are more interested in whether your graduate degree had a research/thesis component to it, or if it was just a course based masters, like the one from NorthEastern (CS dept), for example, where a thesis is optional.

For the most part, masters in science, AFAIK, are seldom course based, and more research oriented - so any university worth its soul will not want its students wasting time/resources on a second master's degree in the same subject - they encourage applicants to consider a PhD instead.

However, if you hold a master's degree in, say, Physics - and want to pursue graduate studies in computer science, this is usually okay.

Could you post details about which subject you've done your masters and bachelors in, whether your graduate degree was had a paper/thesis component, and what you want to pursue in the states? That may help narrow down the answer.