Cannot insert duplicate key in object
<p>I have problem, although i checked primary key does not exist in the database. Additional information:    </p>

<blockquote>
  <p>Additional information: Violation of PRIMARY KEY constraint
  'PK_Yeucaukhachhang'. Cannot insert duplicate key in object
  'dbo.Yeucaukhachhang'. The duplicate key value is (MH01123123).</p>
</blockquote>

<pre><code>using (SqlConnection sqlCon = new SqlConnection(sqlCnt))
{
   for (int i = 0; i &lt; metroGrid2.Rows.Count; i++)
   {
      SqlCommand cmd = new SqlCommand("INSERT INTO
      Yeucaukhachhang(MaKH,MaHang,TenHang,DonViTinh,Dongia,
      SoLuong,Duyet) values('"+ makh.Text+"','"+
      metroGrid2.Rows[i].Cells["mahang"].Value +
      "','"+metroGrid2.Rows[i].Cells["tenhang"].Value+"', '"+ 
      metroGrid2.Rows[i].Cells["donvitinh"].Value+"', '"+ 
      metroGrid2.Rows[i].Cells["dongia"].Value+"', '"+ 
      metroGrid2.Rows[i].Cells["soluong"].Value+"', 'N')", sqlCon);
      SqlCommand cmd1 = new SqlCommand("INSERT INTO DanhMucKhachHang(MaKhachHang,TenKhachHang,DiaChiKhachHang,SDTKhachHang,CMD,masothue,thanhtoan,nganhang,taikhoannganhang,ngaythang,MaHang,TenHang,DonViTinh,Dongia,SoLuong,Duyet) VALUES(@MaKhachHang,@TenKhachHang,@DiaChiKhachHang,@SDTKhachHang,@CMD,@masothue,@thanhtoan,@nganhang,@taikhoannganhang,@ngaythang)", sqlCon);
                 cmd1.Parameters.AddWithValue("@MaKhachHang", makh.Text);
                 cmd1.Parameters.AddWithValue("@TenKhachHang", namekh.Text);
                 cmd1.Parameters.AddWithValue("@DiaChiKhachHang", address.Text);
                 cmd1.Parameters.AddWithValue("@SDTKhachHang", phone.Text);
                 cmd1.Parameters.AddWithValue("@CMD", idkh.Text);
                 cmd1.Parameters.AddWithValue("@masothue", idthue.Text);
                 cmd1.Parameters.AddWithValue("@thanhtoan", deliver.Text);
                 cmd1.Parameters.AddWithValue("@nganhang", bank.Text);
                 cmd1.Parameters.AddWithValue("@taikhoannganhang", idacc.Text);
                 cmd1.Parameters.AddWithValue("@ngaythang", this.datekh.Value);

                 sqlCon.Open();
                 cmd.ExecuteNonQuery();
                 cmd1.ExecuteNonQuery();
                 sqlCon.Close();
                 MessageBox.Show("Thêm thành công, đa chuyển qua xác nhận yêu cầu");
                 LTQL.Home.Home cort = new LTQL.Home.Home();
                 cort.Show();
                 this.Hide();
             }
</code></pre>