function _styleSheetAdded(event, res) {
        var url             = _canonicalize(res.header.sourceURL),
            existing        = styleForURL(res.header.sourceURL),
            styleSheetId    = res.header.styleSheetId,
            duplicate;

        // detect duplicates
        duplicate = _.some(existing, function (styleSheet) {
            return styleSheet && styleSheet.styleSheetId === styleSheetId;
        });
        if (duplicate) {
            return;
        }

        _styleSheetDetails[styleSheetId] = res.header;
        _styleSheetDetails[styleSheetId].canonicalizedURL = url; // canonicalized URL

        exports.trigger("styleSheetAdded", url, res.header);
    }
@private
@param {jQuery.Event} event
@param {header: CSSStyleSheetHeader}