Android Alertdialog

Well, I'm developing an Android application and at the moment, I've encountered a problem for which I can not find a solution, I'll probably be doing something wrong, I hope someone can help me. The problem consists of the following: I have an alertdialog that opens when the application opens and asks for a code. The user enters the code and the application checks the code in the database (mysql) and receives the user data through that code. So far so good. But, when I move from the main activity to a second activity, it repeats the same process again and the alertdialog is opened again to request a code. I want the alertdialog to only appear when starting the app. Can someone help me? Below is the code of the OnCreate method of the main Activity.

//Cria Formulário de pedido
    AlertDialog.Builder builder = new 
AlertDialog.Builder(MainActivity.this);
    //Titulo do AlertDialog
    builder.setTitle("Insira o seu código:");

    // Set up the input
    final EditText input = new EditText(MainActivity.this);

    input.setInputType(InputType.TYPE_CLASS_TEXT | 
InputType.TYPE_CLASS_TEXT);
    builder.setView(input);

    // Set up the buttons
    builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            txtstatus.setText("A verificar código . . .");
            //Recebe Código
            at = input.getText().toString();

                //Guarda o Código e o Url
                at = "at=" + at;
                url_nome_user = "/user_nome.php";

                //Verifica e recolhe os dados do Código
                BackgroundTask backgroundTask = new BackgroundTask();
                backgroundTask.execute(url_nome_user);
                txtstatus.setText("A receber Dados associados . . .");
            }
        }
    });
    builder.setNegativeButton("Cancelar", new 
DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            //Se o Utilizador cancelar
            dialog.cancel();
            Toast.makeText(MainActivity.this,"Falha ao introduzir Código!", 
Toast.LENGTH_SHORT).show();
            txtstatus.setText("Falha ao introduzir Código!");
            finish();
        }
    });
    //Desativar Botão Back
    builder.setCancelable(false);
    //cria o AlertDialog
    AlertDialog alerta = builder.create();
    //Exibe
    alerta.show();

Now in BackgroundTask:

class BackgroundTask extends AsyncTask<String, Void, String>{
    @Override
    protected void onPreExecute() {

    }
    @Override
    protected String doInBackground(String... urls) {
        //Verifica na API o Código de Acesso
        //Dados User
        nome_user = Conexao.postDados(url_nome_user,at);
        return nome_user;
    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }

    @Override
    protected void onPostExecute(String result) {

        //Mostra o Resultado
        Toast.makeText(MainActivity.this,result, Toast.LENGTH_LONG).show();
        MudardeActivity();
    }
  }

public void MudardeActivity(){
 /* Create an Intent that will start the Menu-Activity. */
Intent intent = new Intent(MainActivity.this,Inicio.class);
intent.putExtra("nome_user", nome_user);

MainActivity.this.finish();
startActivity(intent);

}

Can someone help me? Thank you.