node.js - Node js return can not set after headers they are sent -

i'm having problem node.js return-an error on console saying:

error: can not set after headers sent. 

right here part of code returning error:

app.post ('/ register', function (req, res) {     var = usuariocadastro req.body.usuario;     var = emailcadastro req.body.email;     var = telefonecadastro req.body.telefone;     var = senhacadastro req.body.senha;      console.log ("user:" + usuariocadastro);     console.log ("email:" + emailcadastro);     console.log ("telephone" + telefonecadastro);     console.log ("password" + senhacadastro);        pool.query('select * users email = ?',[emailcadastro], function(err, rows, fields) {             if (err) throw err;             (var in rows){ var emailbanco = rows[i].email; }             if(emailcadastro === emailbanco){                 console.log("email já foi cadastrado");                 res.set('content-type', 'application/json');                 res.send({ code: '303', mensagem:'this email signup' });              }else{                 var sql = {usuario:usuariocadastro,email:emailcadastro,senha:senhacadastro};                 pool.query('insert users set ?',[sql], function(err, rows, fields) {                     if (err) throw err;                     console.log("novo usuario cadastrado");                     res.send({ code: '200', mensagem:'sucess, welcome trimtrim' });                  });/*second query*/              }/* else */   

});/* first query*/

i've been researching , saw, expression can send once. need send more times. do?

you can send headers once. aggregate messages , send them after loop.

        connection.release ();         var msg = '';         (var in rows) {             if (emailcadastro === rows [i] .email) {             console.log ("email database:" + rows [i] .email);              if (msg.length>0) msg += '<br/>';             msg += 'e-mail registered: ' + rows[i].email;              errodeform ++;              }          } //          if (msg.length>0) res.json ({code: 303 message: msg});