[Nerull]

Actually, if you base it on what was given as an example for charm person in the rule books (I'd have to unpack the things to find the page number, since I'm moving around rooms in the house right now), the second spell does replace the first. They use charm person as the example, but state that if someone is affected by a spell with a duration, and someone casts an identical spell on that person, then the second spell replaces the first. And it makes sense, otherwise you have players filling all of their second level slots with mirror image, and creating an army of images.