You can do either, but there are some concerns for both options. If you remove the carpeting, you will have to flatten out the glue on the concrete surface. You do not have to remove it completely, but you will have to scrape it smooth. The benefit of laying the new carpet over the old-aside from saving the trouble of scraping off the glue-is that the old carpet will act as an underlay. But you must be absolutely certain that there is no moisture in or under that carpet. If the room already smells musty, then you should remove the old carpet first. If there is no odor, lift up the edge of the carpet in several places and examine the underside of the carpet. If it is dry and odorless, then proceed with laying down the new carpet over the old.