Simple Combobox in Winforms - Firing twice (Use mouse to expand combobox and select a item using keyboard)

Consider a very basic form with a simple combo box

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();
        this.comboBox1.Items.Add("test1");
        this.comboBox1.Items.Add("test2");
        this.comboBox1.Items.Add("test3");
    }

    private void Form1_Load(object sender, EventArgs e)
    {
        this.comboBox1.SelectedIndexChanged += (o, args) => 
        {
            MessageBox.Show("Combo box changed!");
        };
    }
}

I have even changed the eventhandler to the below code. (Based on the linked question. Still the same problem)

this.comboBox1.SelectedValueChanged += (o, args) => 
            {

                MessageBox.Show("Combo box changed!");
            };

Expand the dropdown using mouse and select any item using keyboard.

The combo-box fires twice (The message box appears twice)

Any ideas why?