How to not allow a draggable in a div with an existing that is dragged first using javascript? I only want one draggable in a div?? my script in dragging and dropping image in a div..

function allowDrop(ev) 
    {
      ev.preventDefault();
    }


function drag(ev) 
    {
      ev.dataTransfer.setData("text", ev.target.id);
    }


function drop(ev) 
    {
      ev.preventDefault();
      var data = ev.dataTransfer.getData("text"); 
      ev.target.appendChild(document.getElementById(data));
    }