Add field exist check for alignment field

Author: aspark21

db/upgrade.php

@@ -182,7 +182,10 @@ function xmldb_customcert_upgrade($oldversion) {
         $table = new xmldb_table('customcert_elements');
         $field = new xmldb_field('alignment', XMLDB_TYPE_CHAR, '1', null, XMLDB_NOTNULL, null, 'L', 'refpoint');
 
-        $dbman->add_field($table, $field);
+        // Conditionally launch add field.
+        if (!$dbman->field_exists($table, $field)) {
+           $dbman->add_field($table, $field);
+        }
 
         upgrade_mod_savepoint(true, 2021051702, 'customcert'); // Replace with the actual version number.
     }