SerialPort::flush() sends all the data written to port again and again like its buffer is never cleaned

Following piece of code:

QSerialPort * port = new QSerialPort(this);
port->setPortName("COM1");
port->open(QIODevice::ReadWrite);
port->write("12");
port->write("3");
port->flush();

port->close();
port->open(QIODevice::WriteOnly);

port->write("4");
port->flush();
port->flush();
port->close();

Sends in fact the following string: "12123444".

In my opinion, this looks like SerialPort's buffer isn't cleaned after ultimate submission of its data.