diff --git a/xmlout.js b/xmlout.js
index 72d7f6e..3955ca4 100644
--- a/xmlout.js
+++ b/xmlout.js
@@ -194,6 +194,7 @@ function prefixMessageWithSN(data) {
         sn = sn.replace(/-+/g, '-')
         sn = sn.replace(/^-+/, '');
         sn = sn.replace(/-+$/, '');
+        sn = sn.replace(/;\s*SN=.+?;/, '; ');
         sn = sn.trim();
         return 'SN=' + sn + '; ' + data['MESSAGE'];
     }