Do not insert duplicate values into mysql - mysql

Question

I want to insert users in mysql without duplicate values at the field "User" .how to show a message to the user when he writes a "User" already exist in the table before he complete the rest of the fields and before he clicks on the button Login?
public void Registrar() throws SQLException {
Connection con = null;
ResultSet rs = null;
Statement st = null;
String user;
String pass;
String pass2;
String email;
String country;
user = jTextField1.getText();
pass = jPasswordField1.getText();
pass2 = jPasswordField2.getText();
email = jTextField2.getText();
country = jComboBox1.getSelectedItem().toString();
if (user.isEmpty()) {
jLabel8.setText("Please enter an Username");
} else if (pass.isEmpty()) {
jLabel8.setText("Please enter an password");
} else if (pass2.isEmpty()) {
jLabel8.setText("please you must confirm your password");
} else if (!pass2.equals(pass)) {
jLabel8.setText("passwords do not match");
} else if (email.isEmpty()) {
jLabel8.setText("Please enter an valid email");
} else if (country.equals("Choose your country")) {
jLabel8.setText("Please choose a Country");
} else {
String sql = "INSERT INTO usuarios(User,Pass,Email,Country) "
+ "VALUES('" + user + "','" + pass + "','" + email + "','" + country + "')";
try{
Class.forName("com.mysql.jdbc.Driver");
con = DriverManager.getConnection("jdbc:mysql://localhost/whichmovie", "Asis", "dekrayat24");
System.out.println("Conectado a la base de datos SQLite");
PreparedStatement pstm = con.prepareStatement(sql);
pstm.execute();
pstm.close();
JOptionPane.showMessageDialog(rootPane, "Congratulations you have created your account");
dispose();
Login l = new Login(null, rootPaneCheckingEnabled);
l.setVisible(true);
} catch (Exception e) {
JOptionPane.showMessageDialog(rootPane, "An error has occurred ,please try later");
}
}
}