extensionSystem: Don't load extensions multiple times

Right now if XDG_DATA_DIRS is set to have duplicate entries,
then the extension system will try to load all extensions
more than once.

This commit prevents an extension from getting repeatedly
loaded by checking if its uuid is already registered.

https://bugzilla.gnome.org/show_bug.cgi?id=632477

js/ui/extensionSystem.js

@@ -51,6 +51,12 @@ function loadExtension(dir, enabled, type) {
             return;
         }
     }
+
+    if (extensions[meta.uuid] != undefined) {
+        global.logError(baseErrorString + "extension already loaded");
+        return;
+    }
+
     // Encourage people to add this
     if (!meta['url']) {
         global.log(baseErrorString + 'Warning: Missing "url" property in metadata.json');