Query creates duplicate entries in SQL server

 On click below code creates 4 entries of same data.

 

private void button1_Click(object sender, EventArgs e)

{

SqlCommand cmd = new SqlCommand("insert into class ([className],[mfee],[adfee],[status],[date] )values('" + textBox1.Text + "', '" + textMfee.Text + "', '" + textAdfee.Text + "', '" + comboBox1.SelectedItem.ToString() + "', @Date )", con);

SqlDataAdapter sda = new SqlDataAdapter(cmd);

DateTime ndt = DateTime.ParseExact(textDate.Text, "dd/MM/yyyy", CultureInfo.InvariantCulture);

cmd.Parameters.AddWithValue("@Date", ndt);

con.Open();

cmd.ExecuteNonQuery();

con.Close();

DataTable dt = new DataTable();

sda.Fill(dt);

if (dt.Rows.Count > 0)

{

MessageBox.Show("Class Name already exist");

}

else

{

SqlDataAdapter sda0 = new SqlDataAdapter(cmd);

con.Open();

cmd.ExecuteNonQuery();

con.Close();

DataTable dt0 = new DataTable();

sda0.Fill(dt0);

MessageBox.Show("New Class added");

}