MDL-10523 - Removing DISTINCT from the query will enable the function to work with MSSQL which was having difficulty with the current syntax and should not impact other queries because the user id should already be unique.

group/lib/legacylib.php

@@ -112,7 +112,7 @@ function get_group_users($groupid, $sort='u.lastaccess DESC', $exceptions='',
     if (!empty($extrafield)) {
         $extrafield = ','.$extrafield;
     }
-    return get_records_sql("SELECT DISTINCT $fields $extrafield
+    return get_records_sql("SELECT $fields $extrafield
                               FROM {$CFG->prefix}user u,
                                    {$CFG->prefix}groups_members m
                              WHERE m.groupid = '$groupid'