var check;
function pop(string) {   

  if(check == 1) {
    pop.close();
  }
  pop = window.open("", "uitleg", "height=400,width=350,scrollbars=yes,titlebar=no, status=no, toolbar=no, menubar=no");
  pop.location = string;
  //  pop.document.body.innerHTML = 'jaja';
  check = 1;
  
}