Index: term_reference_tree.js
===================================================================
--- term_reference_tree.js	(revision 589)
+++ term_reference_tree.js	(working copy)
@@ -2,7 +2,7 @@
   Drupal.behaviors.termReferenceTree = {
     attach: function(context, settings) {
       // Bind the term expand/contract button to slide toggle the list underneath.
-      $('.term-reference-tree-button', context).click(function() {
+      $('.term-reference-tree-button', context).once().click(function() { // once() used to avoid the click handler being attached multiple times (otherwise attach is called for each AJAX refresh)
         $(this).toggleClass('term-reference-tree-collapsed');
         $(this).siblings('ul').slideToggle('fast');
       });