查找datatable 中的重復記錄（只查詢一個字段）

                StringBuilder str = new StringBuilder();
                var res = new ResParameter() { code = ResponseCode.exception };


                var result = from isbn in dt.AsEnumerable()
                             group isbn by isbn.Field<string>("isbn") into grp
                             where grp.Count() > 1
                             select grp.Key;


                if (result.Count() > 0)
                {
                    foreach (var item in result)
                    {
                        str.Append(item + ",  ");
                    }
                    res.info ="導入EXCEL中包含重復ISBN'  "+ str.ToString()+"'  請整理后再重新導入";
                    return res;
                }