问Linq- to -SQL:正在尝试创建实体的新版本，但get“无法添加已存在的实体”

EN

[TestModule]
ProductId uniqueidentifier primarykey
ModuleId uniqueidentifier primarykey
LastEdited timestamp primarykey
LastEditor uniqueidentifier
LastEditComment nvarchar(200)
CheckedOutTo uniqueidentifier
... other fields ...

    [Column(IsPrimaryKey=true)]
    public Guid ProductGuid { get; private set; }

    [Column(IsPrimaryKey = true)]
    public Guid ModuleGuid { get; private set; }

    /// <summary>
    /// date of last edit
    /// </summary>
    [Column(IsPrimaryKey=true)]
    public DateTime LastEditDate { get; set; }

    /// <summary>
    /// Guid of last editor
    /// </summary>
    [Column]
    public Guid LastEditorGuid { get; set; }

    /// <summary>
    /// Summary of last changes
    /// </summary>
    [Column]
    public string LastEditNote { get; set; }

    /// <summary>
    /// Current editor. Null if checked in
    /// </summary>
    [Column(CanBeNull=true)]
    public Guid CheckedOutByGuid { get; set; }