_http_outgoing.js:371 throw new Error('Can\'t set headers after they are sent.');

When I run the code then I fill the form the first time, the result is displayed well! but when I refresh the page or click on submit I get this error :

_http_outgoing.js:371 throw new Error('Can\'t set headers after they are sent.');

---

this is the code that I have written:

router.post('/send', function(req, res, next) {
var currentUser=req.body.currentUserAddress
var reciever= req.body.Addressedestinataire;
var amount = req.body.somme;
var data=[{"Spalte":amount}];
var s =JSON.stringify(data[0].Spalte);
var valueToken = JSON.parse(s);
mytoken.transfer.sendTransaction(currentUser,reciever,amount,{from :web3.eth.accounts[0],gas:5000000},function (error, result)
 {

    if(result)
          { 
                let event = mytoken.Transfer();

                 event.watch(function(err, response)
                {
                    if (err) {
                        console.log ("Error="+ err);
                             }
                    if (!err) {
                         let addressEvent= response.address;
                         let addressTo=response.args.to;
                         let addressFrom =response.args.from;
                         Transaction={addressFrom,addressTo,amount,date}
                         db.open(function(err, db) {

                         let collection = db.collection("Transaction");

                         collection.insertOne(Transaction, function(err, result1) {assert.equal(null, err);});
                         db.close();
                                 });
          res.setHeader('Content-Type', 'application/json');
          res.send(JSON.stringify({addressEvent,addressFrom,addressTo,amount}));

                    }
       });

         }
   else{
  console.log(error);  
       }

    });
 });

`

via takampika