diff --git a/lib/core.js b/lib/core.js
index 906a3fd3..c364b113 100644
--- a/lib/core.js
+++ b/lib/core.js
@@ -138,7 +138,7 @@ hook('normalize', function(normalize) {
 
     if (name.match(absURLRegEx)) {
       // defaultJSExtensions backwards compatibility
-      if (this.defaultJSExtensions && name.substr(name.length - 3, 3) != '.js')
+      if (this.defaultJSExtensions && name.substr(name.length - 3, 3) != '.js' && !this.defined[name])
         name += '.js';
       return name;
     }
@@ -146,16 +146,18 @@ hook('normalize', function(normalize) {
     // applyPaths implementation provided from ModuleLoader system.js source
     name = applyPaths(this.paths, name) || name;
 
-    // defaultJSExtensions backwards compatibility
-    if (this.defaultJSExtensions && name.substr(name.length - 3, 3) != '.js')
-      name += '.js';
-
     // ./x, /x -> page-relative
     if (name[0] == '.' || name[0] == '/')
-      return new URL(name, baseURIObj).href;
+      name = new URL(name, baseURIObj).href;
     // x -> baseURL-relative
     else
-      return new URL(name, getBaseURLObj.call(this)).href;
+      name = new URL(name, getBaseURLObj.call(this)).href;
+
+    // defaultJSExtensions backwards compatibility
+    if (this.defaultJSExtensions && name.substr(name.length - 3, 3) != '.js' && !this.defined[name])
+      name += '.js';
+
+    return name;
   };
 });