Chrome extension, javascript: Why is this firing twice?

I have a very very simple bit of code in my (test) Chrome extension:

    function test()
    {
    alert("In test!");
    }

   chrome.tabs.onUpdated.addListener(function(tabid, changeinfo, tab) {
    var url = tab.url;
        if (url !== undefined) {

        test();
    }
   });

My question is, why is test() firing twice? And more importantly, how do I make it fire just once?