diff --git a/loader.js b/loader.js
index ea388adf..cb0f3eac 100644
--- a/loader.js
+++ b/loader.js
@@ -845,9 +845,9 @@
 
   // carefully scoped eval with given global
   var __scopedEval = function(__source, global, __sourceURL, __sourceMappingURL) {
-    eval('with(global) { (function() { ' + __source + ' \n }).call(global); }' + (__sourceMappingURL 
-      ? '\n//# sourceMappingURL=' + __sourceMappingURL 
-      : (__sourceURL ? '\n//# sourceURL=' + __sourceURL : '')));
+    eval('with(global) { (function() { ' + __source + ' \n }).call(global); }'
+      + (__sourceURL && !__source.match(/\/\/[@#] ?(sourceURL|sourceMappingURL)=(.+)/)
+      ? '\n//# sourceURL=' + __sourceURL : ''));
   }
 
 })();