Seçili eleman listede var ise listeye eklemeyen, yoksa ekleyen basit deneme private void btnSag_Click(object sender, EventArgs e) { if (cbList.SelectedIndex>-1) { if (lstList.Items.IndexOf(cbList.SelectedItem)== -1) { lstList.Items.Add(cbList.SelectedItem); } //cbList.Items.RemoveAt(cbList.SelectedIndex); } } private void btnSol_Click(object sender, EventArgs e) { i...