diff --git a/public/res/extensions/markdownExtra.js b/public/res/extensions/markdownExtra.js
index e03e1be3..9e95acdf 100644
--- a/public/res/extensions/markdownExtra.js
+++ b/public/res/extensions/markdownExtra.js
@@ -95,7 +95,8 @@ define([
             var previewContentsElt = document.getElementById('preview-contents');
             editor.hooks.chain("onPreviewRefresh", function() {
                 _.each(previewContentsElt.querySelectorAll('.prettyprint > code'), function(elt) {
-                    hljs.highlightBlock(elt);
+                    !elt.highlighted && hljs.highlightBlock(elt);
+                    elt.highlighted = true;
                 });
             });
         }