Radio button checked changed event fires twice

It's triggering once for the radio button transition from checked to unchecked, and again for the radio button transitioning from unchecked to checked (i.e. any change in checked state triggers the event)

CheckedChanged is raised whenever the Checked property changes. If you select a RadioButton then the previously selected RadioButton is unchecked (fired CheckedChanged), and then the new RadioButton is checked (fired CheckedChanged).

To avoid it, just check if radioButton is checked

for example:

private void radioButton1_CheckedChanged(object sender, EventArgs e)
{
    if (radioButton1.Checked)
        //your code
}

As the other answerers rightly say, the event is fired twice because whenever one RadioButton within a group is checked another will be unchecked - therefore the checked changed event will fire twice.

To only do any work within this event for the RadioButton which has just been selected you can look at the sender object, doing something like this:

void radioButtons_CheckedChanged(object sender, EventArgs e)
{
    RadioButton rb = sender as RadioButton;
    if (rb != null)
    {
        if (rb.Checked)
        {
            // Only one radio button will be checked
            Console.WriteLine("Changed: " + rb.Name);
        }
    }
}