C#Visual Studio违反了SQL Server数据库的PK

我正在为我的学校项目制定奖励系统，我需要帮助解决我违反PK的问题。有人知道为什么会发生这种情况吗？

这是我的代码（points.cs）

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
public class Points
{
    //string _connStr = Configuration.ConnectionStringSettings _connStr;
    string connStr = ConfigurationManager.ConnectionStrings["EBizDBContext"].ConnectionString;
    private string _username = "";
    private int _trans_no = 0;
    private string _date = "";
    private int _points_added = 0;
    private int _points_deducted = 100;
    private string _description = "";
    public Points()
    {
    }
    public Points(string username, int transNo, string date, int points_added, int points_deducted)
    {
        _username = username;
        _trans_no = transNo;
        _date = date;
        _points_added = points_added;
        _points_deducted = points_deducted;
    }
    public Points(string username, string date, int points_added, int points_deducted) : this(username, 0, date, points_added, points_deducted)
    {
    }
    public Points(int trans_no) : this("", trans_no, "", 0, 0)
    {
    }
    public Points(string date, int points_added, int points_deducted)
    {
        // TODO: Complete member initialization
        this._date = date;
        this._points_added = points_added;
        this._points_deducted = points_deducted;
    }
    public Points(int trans_no, string date, int points_added, int points_deducted)
    {
        this._trans_no = trans_no;
        this._date = date;
        this._points_added = points_added;
        this._points_deducted = points_deducted;
    }
    public int trans_no
    {
        get { return _trans_no; }
        set { _trans_no = value; }
    }
    public string date
    {
        get { return _date; }
        set { _date = value; }
    }
    public int points_added
    {
        get { return _points_added; }
        set { _points_added = value; }
    }
    public int points_deducted
    {
        get { return _points_deducted; }
        set { _points_deducted = value; }
    }
    public string username
    {
        get { return _username; }
       set { _username = value; }
    }
    public Points getPoints(string username)
    {
         Points pointsDetails = null;
         int trans_no, points_added, points_deducted;
         string date;
         string queryStr = "SELECT * FROM Points WHERE username = 'ad';";
         SqlConnection conn = new SqlConnection(connStr);
         SqlCommand cmd = new SqlCommand(queryStr, conn);
         cmd.Parameters.AddWithValue("@username", username);
         conn.Open();
         SqlDataReader dr = cmd.ExecuteReader();
         if (dr.Read())
         {
             trans_no = int.Parse(dr["trans_no"].ToString());
             date = dr["date"].ToString();
             points_added = int.Parse(dr["points_added"].ToString());
             points_deducted = int.Parse(dr["points_deducted"].ToString());
             pointsDetails = new Points(username, trans_no, date, points_added, points_deducted);
        }
        else
        {
            pointsDetails = null;
        }
        conn.Close();
        dr.Close();
        dr.Dispose();
        return pointsDetails;
    }
    public List<Points> getPointsAll()
    {
        List<Points> pointsList = new List<Points>();
        string username, date;
        int trans_no, points_added, points_deducted;
        string queryStr = "SELECT * FROM Points Order By username";
        SqlConnection conn = new SqlConnection(connStr);
        SqlCommand cmd = new SqlCommand(queryStr, conn);
        conn.Open();
        SqlDataReader dr = cmd.ExecuteReader();
        while (dr.Read())
        {
            username = dr["username"].ToString();
            trans_no = int.Parse(dr["trans_no"].ToString());
            date = dr["date"].ToString();
            points_added = int.Parse(dr["points_added"].ToString());
            points_deducted = int.Parse(dr["points_deducted"].ToString());
            Points a = new Points(username, trans_no, date, points_added, points_deducted);
            pointsList.Add(a);
        }
        conn.Close();
        dr.Close();
        dr.Dispose();
        return pointsList;
    }
    public List<Points> getPointsByUsername()
    {
        List<Points> pointsList = new List<Points>();
        string date;
        int trans_no, points_added, points_deducted;
        string queryStr = "SELECT trans_no, date, points_added, points_deducted FROM points WHERE username = 'pp';";
        SqlConnection conn = new SqlConnection(connStr);
        SqlCommand cmd = new SqlCommand(queryStr, conn);
        cmd.Parameters.AddWithValue("@username", username);
        conn.Open();
        SqlDataReader dr = cmd.ExecuteReader();
        while (dr.Read())
        {
             trans_no = int.Parse(dr["trans_no"].ToString());
             date = dr["date"].ToString();
             points_added = int.Parse(dr["points_added"].ToString());
             points_deducted = int.Parse(dr["points_deducted"].ToString());
             Points b = new Points(trans_no, date, points_added, points_deducted);
             pointsList.Add(b);
        }
        conn.Close();
        dr.Close();
        dr.Dispose();
        return pointsList;
    } // end of retrieve
    public int PointsInsert1()
    {
        string msg = null;
        int result = 0;
        string queryStr = "INSERT INTO points(username, trans_no, date, points_deducted)"
        + "VALUES(@username, @trans_no, @date,@points_deducted);" + "SELECT @@IDENTTY AS int32;";
        SqlConnection conn = new SqlConnection(connStr);
        SqlCommand cmd = new SqlCommand(queryStr, conn);
        cmd.Parameters.AddWithValue("@username", this.username);
        cmd.Parameters.AddWithValue("@trans_no", this.trans_no);
        cmd.Parameters.AddWithValue("@date", this.date);
        cmd.Parameters.AddWithValue("@points_deducted", this.points_deducted);
        conn.Open();
        result += cmd.ExecuteNonQuery();
        conn.Close();
        return result;
    }
}

点DB表

CREATE TABLE [dbo].[points] 
(
    [username]        NVARCHAR(20) NOT NULL,
    [trans_no]        INT IDENTITY(1, 1)    NOT NULL,
    [date]            DATETIME     NOT NULL,
    [points_added]    INT          NULL,
    [points_deducted] INT          NULL,
    PRIMARY KEY CLUSTERED ([trans_no] ASC)
);

我希望我能够保存点击btn_done时扣除的分数。然而，我不知道如何使trans_no成为最新的

public int PointsInsert1()
    {
        string msg = null;
        int result = 0;
        string queryStr = "INSERT INTO points(username,  date, points_deducted)"
        + "VALUES(@username, @date,@points_deducted);" + "SELECT @@IDENTTY AS int32;";
        SqlConnection conn = new SqlConnection(connStr);
        SqlCommand cmd = new SqlCommand(queryStr, conn);
        cmd.Parameters.AddWithValue("@username", this.username);
        cmd.Parameters.AddWithValue("@date", this.date);
        cmd.Parameters.AddWithValue("@points_deducted", this.points_deducted);
        conn.Open();
        result += cmd.ExecuteNonQuery();
        conn.Close();
        return result;
    }