How can you bind an event handler only if it doesn't already exist?

Unbind existing event handlers before you bind the new ones. This is really straightforward with namespaced events ^[docs]:

$(document)
  .off('.upload') // remove all events in namespace upload
  .on({
      'dragenter.upload': function(e) {
          e.stopPropagation();
          e.preventDefault();
      },
      'dragover.upload': function(e) {
          e.stopPropagation();
          e.preventDefault();
      },
      // ...
  });