how to validate repeated values in each cell using datagridivew in csharp

if (e.KeyChar == 118 || e.KeyChar == 86)// v,V to paste
                {
                    String Msg = "";
                    if (CopyString != "" && CopyString != null)
                    {
                        int irow = Convert.ToInt16(datagridView.CurrentCellAddress.Y.ToString());
                        for (int i = 2; i < datagridView.Columns.Count; i++)
                        {
                            if (datagridView[i, irow].Value != "" && datagridView[i, irow].Value != null)
                            {
                                if (datagridView[i, irow].Value.ToString().Trim() == CopyString.ToString().Trim())
                                {
                                    Msg = "Course is already entered in " + datagridView.Columns[i].HeaderText;
                                }
                            }
                        }
                        //if (Msg.ToString() != null)
                        //{
                        //    MessageBox.Show(Msg.ToString());
                        //}
                        //else
                        {
                            datagridView.CurrentCell.Value = CopyString.ToString();
                        }
                    }
                    e.Handled = true;
                }