Index: lams_central/web/includes/javascript/groupDisplay.js
===================================================================
diff -u -racc8d2acf5b6b0002e0c8129947040a779ab4077 -rce9bbbe533e654903bd902434cc908daf7568582
--- lams_central/web/includes/javascript/groupDisplay.js	(.../groupDisplay.js)	(revision acc8d2acf5b6b0002e0c8129947040a779ab4077)
+++ lams_central/web/includes/javascript/groupDisplay.js	(.../groupDisplay.js)	(revision ce9bbbe533e654903bd902434cc908daf7568582)
@@ -74,6 +74,10 @@
 						success: function(html) {
 							jQuery(courseBg).append(html);
 							registerToolTip(this);
+							// unregister and re-register thickbox for this group in order to avoid double
+							// registration of thickbox for existing elements (i.e. group 'add lesson' link)
+							$('a.thickbox'+jQuery(courseBg).attr("id")).unbind("click");
+							tb_init('a.thickbox'+jQuery(courseBg).attr("id"));
 						}
 					});
 				}