How to avoid duplication when inserting

private void button4_Click(object sender, EventArgs e)

{

dataGridView2.Rows.Clear();

Program.cmd.CommandText = "select * from bon_reception_marche where Date_reception between '" + dateTimePicker1.Value.Date + "' and '" + dateTimePicker2.Value.Date + "' and Id_marche in (select TOP 1 Id_marche from marche where Num_marche = '"+textBox1.Text+"')";

Program.dr = Program.cmd.ExecuteReader();

while (Program.dr.Read())

{

dataGridView2.Rows.Add(Program.dr[0],Program.dr[2], Program.dr[3], Program.dr[5], Program.dr[6], Program.dr[7], Program.dr[8], Program.dr[9], Program.dr[10], Program.dr[11], Program.dr[12]);

}

Program.dr.Close();

}

private void dataGridView2_CellClick(object sender, DataGridViewCellEventArgs e)

{

if (e.ColumnIndex == 11/*myColumn*/ && e.RowIndex >= 0 /*myRow*/)

{

button1.Enabled = true;

}

}

private void button1_Click(object sender, EventArgs e)

{

int colIndex = dataGridView2.Columns["CheckBox"].Index;

try

{

var rows = dataGridView2.Rows

.Cast()

.Where(row => row.Cells[colIndex].Value != null)

.Where(row => (bool)row.Cells[colIndex].Value)

.ToList();

foreach (DataGridViewRow row in rows)

insertRowData(row);

MessageBox.Show("c'est ajouté avec succés");

}

catch (FormatException)

{

MessageBox.Show("Only input numbers into the table!",

"Only Numbers", MessageBoxButtons.OK);

}

catch (Exception)

{

MessageBox.Show("There was an error while saving!",

"Error", MessageBoxButtons.OK);

}

}

private void insertRowData(DataGridViewRow row)

{

double montantValue = Convert.ToDouble(row.Cells["Column7"].Value);

int id_br_value = Convert.ToInt32(row.Cells["Column11"].Value);

string check;

if (checkBox1.Checked == true)

{

check = "O";

}

else

{

check = "N";

}

Program.cmd.Parameters.Clear();

Program.cmd.CommandText = "insert into attachement_marche (Id_bon_reception_marche,Id_marche,Num_attachement,Date_debut,Date_fin,Flag_dernier,Montant,User_create,Date_create) values ( " + id_br_value + ",(select TOP 1 Id_marche from marche where Num_marche = '" + textBox1.Text + "'),'" + textBox3.Text + "','" + dateTimePicker1.Value.Date + "','" + dateTimePicker1.Value.Date + "','" + check + "'," + montantValue + ",'" + values.username + "','" + DateTime.Now.Date + "')";

Program.cmd.ExecuteNonQuery();

}