Can't set headers after they are sent in get request

router.get('/getpostcode', function(req, res){
    console.log(req.query.suburb);

    var options = {
        url: 'http://apiurl?suburb=' + req.query.suburb + "&state=" + req.query.state ,
        headers: {
            'auth-key': key
        }
    }

    request(options, callback);

    function callback(error, response, body){
        if(!error && response.statusCode == 200){
            info = JSON.parse(body);
            info = info.localities.locality;
            if( Object.prototype.toString.call( info ) == '[object Array]' ){

                for ( var x = 0 ; x < info.length ; x++ ){
                    var locx = info[x].location;
                    var qsuburb = req.query.suburb;

                    if( locx == qsuburb.toUpperCase() ){
                        res.send( { 'postcode': info[x].postcode } );
                    }
                }

            } else if (Object.prototype.toString.call( info ) != '[object Array]') {

                var locx = info.location;
                var qsuburb = req.query.suburb;
                if ( locx == qsuburb.toUpperCase() ){
                    res.send( { 'postcode': info.postcode } );
                }
            }            
        }
    }
});

So, I am trying to request data from an API and then based on that data, I am then sending some data back. My code is as above.

Unfortunately, in the callback function, when I am running a loop to find a specific element that I will then send back to the client, when sending that element to client, I am getting the error as shown in the title.

This does not occur when there is no loop and I simply send one element of the data back.

Any ideas on what this could be?

You are sending response more than one for one request, write res.send or JSON outside the loop.

for (var x = 0; x < info.length; x++) {
    var locx = info[x].location;
    var qsuburb = req.query.suburb;
    var postcodes = [];
    if (locx == qsuburb.toUpperCase()) {
        postcodes.push({
            'postcode': info[x].postcode
        });
    }
    res.json(postcodes);
}