Index: mod/glossary/backup/moodle2/restore_glossary_stepslib.php
===================================================================
--- mod/glossary/backup/moodle2/restore_glossary_stepslib.php	(revision 16884)
+++ mod/glossary/backup/moodle2/restore_glossary_stepslib.php	(revision )
@@ -66,7 +66,13 @@
         if (!in_array($data->displayformat, $formats)) {
             $data->displayformat = 'dictionary';
         }
+        if (!empty($data->mainglossary) and $data->mainglossary == 1 and
+            $DB->record_exists('glossary', array('mainglossary' => 1, 'course' => $this->get_courseid()))) {
 
+            // Only allow one main glossary in the course
+            $data->mainglossary = 0;
+        }
+
         // insert the glossary record
         $newitemid = $DB->insert_record('glossary', $data);
         $this->apply_activity_instance($newitemid);