In Wpf App Combobox Repeats It First Value Twice, When I Set Selectedindex=0;

I m just simply populating items in combobox at Window_Loaded event
private void Populatecombo()
{
cbProIcon.Items.Add("Extra Controls");
cbProIcon.Items.Add("Home");
cbProIcon.Items.Add("LogOut");
cbProIcon.Items.Add("Exit");
cbProIcon.Items.Add("Sleep");
cbProIcon.Items.Add("Shutdown");
cbProIcon.Items.Add("Restart");
cbProIcon.Items.Add("Lock");
cbProIcon.Items.Add("Logg Off");
cbProIcon.Items.Add("Hibernate");
cbProIcon.SelectedIndex = 0;
}

then on SelectionChanged event m providing some logic

if (cbProIcon.SelectedIndex == 1)
{
//Home
txtcheck.Text += "Home Button Now Clicked now.";
cbProIcon.SelectedIndex = 0;// m setting Index 0 so that Extra Control should always selected.
}
else if (cbProIcon.SelectedIndex == 2)
{
//LogOut
this.Close();
welcome.Show();
}

& so on
but on window when i select combobox it display like this
https://www.dropbox.com/s/azkclhuosm3u45t/combobox.jpg?dl=0

i want to display Extra Control to be display once.

yeah -SA
actually in combobox what ever value i place in it. it show that value twice
in the case of reply i have posted the source code according to that Extra Control is displayed twice first in combobox when i select it & second in dropdownlist.
so how i can remove/hide the Extra Control value which is displayed in dropdownlist,but not in combobox where user select the combobox