You would have to read in the entire list of xmls stored in your form library (into a secondary data connection). Then, based on the username and date, generate the form (xml) name temporarily (store it in a field). Then try to find a match between the current form's filename and the list of existing filenames. If there is a match, display a warning telling the user that a form already exists. This can probably be accomplished in rules, and if not, then code. I'm not sure if it's possible to detect this and then immediately switch to the existing xml, but that would be an interesting idea. It sounds pretty ambitious, so if you can accomplish it, let us know!