check existing ID before changing ID.

dream/platform/static/src/dream.js

@@ -135,6 +135,11 @@
             $(this).dialog("close");
           },
           Validate: function () {
+	    var new_id = $("#id").val();
+	    if (new_id !== element_id && $('#' + new_id).length > 0) {
+	      alert('This ID is already used.');
+	      return;
+	    };
             var data = {}, prefixed_property_id, property_element;
             var updateDataPropertyList = function (property_list, data,
               prefix) {