how to add new record in datagridview in c#

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C#

hello ,
i m creating the purchase bill
i want to add the record of purchase item in gridview1...
my problem is

when i add the item 1st time in datagridview1 it show 1row..
again i m add the another item 2nd time it add it two times.. the another record
and for 3rd time it add the record 3 time...
i want the record should be add only one time...
how can i do that...
pls tell me what changes should i make to run the program correctly..

my code is given below
thanks.........

void dgv_CellContentDoubleClick(object sender, DataGridViewCellEventArgs e)
       {
           textBox1.Text = dgv.Rows[e.RowIndex].Cells[0].Value.ToString();
           dgv.Hide();
           cn.Open();
           string query = "select part_no,part_name,mrp from part where part_no='" + textBox1.Text + "'";
           dt = new DataTable();
           ds = new DataSet();
           da = new SqlDataAdapter(query, cn);
           da.Fill(ds);

           //Close the Connection
           cn.Close();
           dataGridView1.Rows.Add(1);
           int r = dataGridView1.Rows.Count;

           for (int i = 0; i < ds.Tables[0].Rows.Count; i++)
           {
               //dataGridView1.Rows.Add(1);
               dataGridView1.Rows[r].Cells["gv_part_no"].Value = ds.Tables[0].Rows[i]["part_no"].ToString();
               dataGridView1.Rows[r].Cells["gv_part_name"].Value = ds.Tables[0].Rows[i]["part_name"].ToString();
               dataGridView1.Rows[r].Cells["gv_pono"].Value = "";
               dataGridView1.Rows[r].Cells["gv_docqty"].Value = 1;
               dataGridView1.Rows[r].Cells["gv_rate"].Value = ds.Tables[0].Rows[i]["mrp"].ToString();
               dataGridView1.Rows[r].Cells["gv_amount"].Value = "";
           }