datagridview checking records

To prevent duplicate records while the grid is being edited, handle RowValidating. In the handler, check that the record is unique and do not allow the user to exit the row unless it is. That way, your grid can only ever contain unique records.
Perform the same check in the Validating event (grid validation) in case you are creating a duplicate when you click save (although the RowValidating event will fire, in this case it won't stop the user exiting the row).

If you want to allow duplicate records until Save is clicked, just do verification in the Validating event handler. Then the user cannot leave the grid until the duplicate is deleted.

I've assumed that you have the knowledge to check for duplicates. You need to parse the DataTable to check that.

  var rowCollection = dataGridView1.Rows.OfType<datagridviewrow>().Where(r => !r.IsNewRow).ToList();

            foreach (DataGridViewRow row in dataGridView1.Rows)
            {
                if (row.IsNewRow)
                {
                    rowCollection.ForEach(f =>
                        {
                            //code to check on duplication
                        });
                }
            }

</datagridviewrow>

this method check the already added product in the datagrid

if the product exist then method return false , else return true....

public bool Check_Product_Exists()
      {
          bool result = true;
          string product = "";
          for (int b = 0; b < grd_dtls.Rows.Count - 1; b++)
          {
             
              for (int c = 0; c < grd_dtls.Rows.Count - 1; c++)
              {
                  if (c != b)
                  {
                      if (product.Equals(grd_dtls.Rows[c].Cells[0].Value))
                      {
                          grd_dtls.Rows[c].Selected = true;
                          MessageBox.Show("You have selected a product twice.");
                          return result = false;
                      }
                  }
              }
          }
          return result;
      }

use this code put your grid name instead of mine