C# 带有附加字段的映射表codefirst

我先用代码，我有这个映射表

 public class OpirtiunitiesSourcerUsers
    {
        [Key, Column(Order = 0)]
        [ForeignKey("SourcedUser")]
        public int SourcedUserID { get; set; }
        [Key, Column(Order = 1)]
        [ForeignKey("SourcedPartner")]
        public int SourcedPartnerID { get; set; }

        public virtual SourcedUser SourcedUser { get; set; }
        public virtual SourcedPartner SourcedPartner { get; set; }

        public string ApproversRole { get; set; }
    }

当我尝试添加新的OpartiunitiesSourcerUsers时，引发了异常 一个实体对象不能被多个IEntityChangeTracker实例引用。

OpirtiunitiesSourcerUsers opp1 = new OpirtiunitiesSourcerUsers()
                                {
                                    SourcedUser = someUser,
                                    ApproversRole = "someRole",
                                    SourcedPartner = somePartner
                                };
                                context.OpirtiunitiesSourcerUsers.Add(opp1);