Chrome extension: access one of many values with the same key

I'm trying to write a Google Chrome extension and I am using the chrome.storage API.

I am storing the url of a page using:

    chrome.storage.local.set({'url': document.URL});

But this function is called multiple times, and so in the storage there are multiple different URL's with the same 'url' key.

To get the value I use:

chrome.storage.local.get('url', function (urlResult) {
url = urlResult.url;
document.getElementById("urlInfo").ierHTML = url;
});

But this will retu only the last of the values stored in the key. So for example, if I visit http://stackoverflow.com/ first and https://github.com/ second, my code will only retu the github address. Is it possible to access the stackoverflow value while having multiple values for the same key?

Thanks!