[asettico]

Philip, the plugin that Kevin talked about is not a browser, or client side, plugin, but a forum, or server side, plugin. Only who has administration access to the program files implementing the forum can change the configuration, adding or deleting any plugins.

The double message you posted probably depends on a timed out session, but for some reason the same POST request has been submitted by your browser.

Unfortunately, my reply can't fix anything, it just explains a little bit from where that error message come from.