We are not sure if you are talking about ceramic, 12 inch vinyl, or sheet vinyl. In most cases the tile should come up to give you the best job possible. If the tile that is down uses a black glue, this is a whole new ball of wax. Here are a few reasons why I think the existing tile should come up. There is a wax on the existing floor and the new one won't stick right, the tiles are cupped and the cupping will show thru the new floor, the new tiles will be stuck to the existing tiles and if any are loose they will both come up.