How do validate combobox

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C#

HI every body!
in my project i am adding union council and Village. Now i want to validate it. I mean that if i add viilage council first time. and if i add it 2 or three time it has adding to database. i want to to not add this it only add one time.

What I have tried:

This is my Code


public partial class VillageCouncil : Form
{
public VillageCouncil()
{
InitializeComponent();
}
Dataaccess da = new Dataaccess();
//SqlDataReader reader;
private void VillageCouncil_Load(object sender, EventArgs e)
{
string q = string.Format("Select ID,UCouncilName from UnionCouncil");
DataTable dt = new DataTable();
SqlDataAdapter sd = new SqlDataAdapter(q, da.open());
sd.Fill(dt);
foreach (DataRow dr in dt.Rows)
{
comboBox1.Items.Add(dr[1].ToString());
}
//dt.Columns.Add("ID", typeof(string));
//dt.Columns.Add("UCouncilName", typeof(string));
//dt.Load(reader);

comboBox1.ValueMember = "ID";
comboBox1.DisplayMember = "UCouncilName";
comboBox1.DataSource = dt;


}

private void button1_Click(object sender, EventArgs e)
{
string q = string.Format("insert into VillageCouncil(VCouncilName,UCName) values ('{0}','{1}')",tbVCName.Text.Trim(),comboBox1.Text.Trim());//.SelectedItem.ToString());
SqlCommand sc = new SqlCommand(q, da.open());
sc.ExecuteNonQuery();
}
}
}