diff --git a/jsm.inc.php b/jsm.inc.php
index fd71061..6ef6ac2 100644
--- a/jsm.inc.php
+++ b/jsm.inc.php
@@ -1565,6 +1565,8 @@ class JSM_ArgsParser
     {
       $this->skip_whitespace();
       $value = $this->parse_arg_value();
+      if(isset($out[$name]))
+        throw new Exception("Argument '{$name}' is already defined in def macro");
       $out[$name] = $value;
       $this->skip_whitespace();
       $ch = $this->next();