Just don't repeat the same record please!

Question

Hi!

Please I am trying to don't allow repetition of records. But I cannot resolv it.
It says it's saved but he didnt (the same datas or anothers when I open my sql server table I didnt find the record that was saved as he said).

conn = new SqlConnection(connstr);

                comm = new SqlCommand();

                conn.Open();

                SqlParameter data = new SqlParameter("@data", SqlDbType.NChar);

                SqlParameter classe = new SqlParameter("@classe", SqlDbType.NChar);

                

                comm.Parameters.Add(data);

                comm.Parameters.Add(classe);

                       

                data.Value = DateTime.Today;

                classe.Value = codigoBarraStocks.Text;

                

                comm.Connection = conn;

        

                comm.CommandText = "select count(*) from stocks where classe = @classe";

                int count = (int)comm.ExecuteScalar();

                if (count == 0)

                try

                {

                    comm.ExecuteNonQuery();

                    MessageBox.Show("Saved succefully");

                    

                }

                catch (Exception)

                {

                    MessageBox.Show("Not saved");

                }

                finally

                {

                    conn.Close();

                }