try unsetting input data

booking.variety_form.inc

@@ -53,6 +53,14 @@ function booking_variety_regn_form($node, &$form_state)
   //for each entry in the variety timeslot table, create a new form select item
   foreach($result as $timeslot) {
     $fieldname = 'select-variety-' . $timeslot->tid;
+
+    // Make sure we get rid of any stale data
+    if (isset($form_state['input']['variety-sessions'][$fieldname])) {
+      watchdog('booking_debug', 'Unsetting stale @name data, was: <pre>@info</pre>', 
+        array('@name' => $fieldname, '@info' => print_r($form_state['input'], true)));
+      unset($form_state['input']['variety-sessions'][$fieldname]);
+    }
+
     //create the form element for this timeslot
     $form['variety-sessions'][$fieldname] = array(
       '#type' => 'select',