asynchronous behavior of nodejs - node.js

Question

I am new to node.js and I try to render different pages based on some conditions, however, I sometimes meet the problem "Error [ERR_HTTP_HEADERS_SENT]: Cannot set headers after they are sent to the client", I think this is because of the asynchronous behavior of node.js. How can I fix this problem?
I tried setTimeout but I think that's not the right way to do that.
The following part is the section where I met this issue:
/*
signup err code:
0 stand for no errors,
1 stand for confirmPassword and password not match
2 stand for user already exist
*/
app.post('/signup',(req, res) => {
//initialize the signupErr to be 0
signupErr = 0;
//get the data from the post request
let username = req.body.username;
let email = req.body.email;
let password = req.body.password;
let confirmPassword = req.body.confirmPassword;
let organization = req.body.organization;
/*first check the twice entered password, if not match set error number to 1 for the server to render the corresponding error message*/
if (password!=confirmPassword){
signupErr = 1;
res.redirect('/signup');
}
//form the query to consult the database to check if user exists
let queryUsername = "SELECT password FROM User WHERE Username = " + "'" + username + "'";
let queryEmail = "SELECT email FROM User WHERE Email = " + "'" + email + "'";
// check if username exists
connection.query(queryEmail,(err,results) => {
if(err){
console.log("Errore login: " + err);
} else if(results.length!=0) {
signupErr = 2;
res.redirect('/signup');
}
});
//check if email exists
connection.query(queryUsername,(err,results) => {
if(err){
console.log("Errore login: " + err);
} else if(results.length!=0) {
signupErr = 2;
res.redirect('/signup');
}
});
// form the insertion query if no err found
let queryInsert = "INSERT INTO `PIA`.`User` (`Username`, `Email`, `Password`,`Organization`) VALUES ('"+username +"', '"+ email +"', '"+password +"', '"+organization + "')";
// insert the data into the database
connection.query(queryInsert,(err,results) => {
if(err){
console.log("Errore login: " + err);
} else{
console.log("Inserted");
res.render('main');
}
});
});
The insertion part will still run sometime after the redirect.
I'll be really grateful if you can give me any ideas on how to solve that.