await DisplayAlert pop up when I press back button on Android

Hi my friends.
This is my first thread, so have patience with me and with my poor english.
I've this situation in a ContentPage
var buttonLogout = new Button ();
buttonLogout.Text = "Logout";
buttonLogout.Clicked += async (sender, e) => {
var answer = await DisplayAlert("Logout", "Terminare il turno di lavoro?", "Si", "No");
// if(answer)
//
};

If I press "logout" button on my page, the "Terminare..." message appears.
If I press back button on Android Simulator (after the "logout" button), the "Terminare..." message appears again....
Why does it appear me twice?

Thanks
Alessandro