asp.net mvc / Sql 双重寄存器

我做了一个新的MVC项目，我在数据库中做了插入函数。我的问题是，当我将模型输入数据库时，它会记录两次。尝试调试，但不幸的是找不到错误。下面的代码是否有任何遗漏？

 public class DbCode
    {
        protected SqlConnection conn;
        public bool Open(string Connection = "MyDb")
        {
            conn = new SqlConnection(@WebConfigurationManager.ConnectionStrings[Connection].ToString());
            try
            {
                var b = true;
                if (conn.State.ToString() != "Open")
                {
                    conn.Open();
                }
                return b;
            }
            catch (SqlException ex)
            {
                return false;
            }
        }
        public bool Close()
        {
            try
            {
                conn.Close();
                return true;
            }
            catch (Exception ex)
            {
                return false;
            }
        }
        public int ToInt(object s)
        {
            try
            {
                return Int32.Parse(s.ToString());
            }
            catch (Exception)
            {
                return 0;
            }
        }
        public int DataInsert(string sql)
        {
            int LastID = 0;
            string query = sql + ";SELECT @@Identity;";
            try
            {
                if (conn.State.ToString() == "Open")
                {
                    SqlCommand cmd = new SqlCommand(query, conn);
                    cmd.ExecuteNonQuery();
                    LastID = this.ToInt(cmd.ExecuteScalar());
                }
                return this.ToInt(LastID);
            }
            catch
            {
                return 0;
            }
        }
    }

 public class StudentModel
    {
        [Required]
        [StringLength(5)]
        public string productname { get; set; }
        [Required]
        [StringLength(5)]
        public string quantity { get; set; }
        [Required]
        [StringLength(5)]
        public string price { get; set; }
    }

控制器

 public class StudentController : Controller
    {
        protected DbCode DB = new DbCode();
        public ActionResult Index()
        {
            return View();
        }
        [HttpPost]
        [ValidateAntiForgeryToken]
        public ActionResult SaveDataStudent(StudentModel student)
        {
            if (ModelState.IsValid)
            {
                DB.Open();
                int i = DB.DataInsert("INSERT INTO tblproducts(productname,quantity,price) VALUES('" + student.productname + "','" + student.quantity + "','" + student.price + "')");
                if (i > 0)
                {
                    ModelState.AddModelError("Success", "Save Success");
                }
                else
                {
                    ModelState.AddModelError("Error", "Save Error");
                }
                DB.Close();
            }
            return RedirectToAction("Index", "Student");
        }
    }

学生视图

@model MVC5.Models.StudentModel
@{
    ViewBag.Title = "Index";
}

@using (Html.BeginForm("SaveDataStudent", "Student", new { @id = "Form" }, FormMethod.Post))
{
    @Html.ValidationSummary();
    @Html.AntiForgeryToken();
    @Html.LabelFor(m => m.productname);
    @Html.TextBoxFor(m => m.productname);<br/>
    @Html.LabelFor(m => m.quantity);
    @Html.TextBoxFor(m => m.quantity);<br />
    @Html.LabelFor(m => m.price);
    @Html.TextBoxFor(m => m.price);<br />
    <input type="submit" value="Save" name="Save" />
}

在 Web 配置中添加了连接字符串