I would simply suggest you this condition, it's not optimal but it works the way you want :
bool contains = false;
for (int i = 0; i < lbxUnsortedList.Items.Count; i++)
{
if (lbxUnsortedList.Items[i].ToString().ToLower() == this.tbxAddWord.Text.ToString().ToLower())
{
contains = true;
}
}
if (!contains)
{
//your code
}
else
{
MessageBox.Show("The word that you have added is already on the list");
tbxAddWord.Text = "";
tbxAddWord.Focus();
}