javascript客户端代码：
 function longConnect(){
        var source = new EventSource('/contact/comet');
        source.onmessage=function(event)
        {
            console.log(2);
            console.log(event.data);
        };
    }
 服务端代码
exports.comet = function(req, res, next){
        res.set('Content-Type', 'text/event-stream');

        setInterval(function(){

            res.send("1");
        },3000);

    };

报错：
http.js:690
throw new Error('Can\'t set headers after they are sent.');

不能重复设置 那express框架下该怎么做？