QDoubleSpinBox and QSpinBox valueChanged called twice

If I do something slowly action (>500 ms) in valueChanged or textChanged, when I clicked up-down button one time, this slot will call twice, and SpinBox jumps twice the step value too.

void MainWindow::on_doubleSpinBox_valueChanged(double arg1) 
{ 
    Sleep(600); 
    qDebug()<<"called..."; 
}

But if I delete Sleep(600);, this slot will only call one time.

void MainWindow::on_doubleSpinBox_valueChanged(double arg1) 
{ 
    qDebug()<<"called..."; 
}

 
I build it in Win32/Release.