It might be time to broaden your net a bit. An insert like that should insert just one row, that is pretty fundamental to the way SQL works. My suspicion is that your code is being executed more than once. For example, if your page submits to itself, it might be inserting an empty row when you view the form, and a full one when you submit it.
You might want to log messages to a file to get a better idea of what is going on. (Searching for PHP logging will pull up some good tutorials.)