[eluser]tobben[/eluser]
You could maybe use some javascript. Something like (a bit pseudo and real code here..) :
HTML
<input type="file" name="files" id="files" />
Code:
var fileNameToCheck = '';
$(function(){
$('#files').change(function(){
fileNameToCheck = $(this).val();
// Do some Ajax-stuff to a PHP script that checks the existens of a given filename/filepath
// Return value and show a response to the user "OMG! Duplicate file!!!!"
});
});