项目中更新一张表，数据有新增的也有修改的，代码如下：

//新增

db.Order.AddRange(newOrderList);

//修改

db.Order.Attach(order);
db.Entry(order).State = System.Data.Entity.EntityState.Modified;

在“db.Order.Attach(order);”这句代码抛异常，异常信息如下：

Attaching an entity of type 'Model.Order' failed because another entity of the same type already has the same primary key value. This can happen when using the 'Attach' method or setting the state of an entity to 'Unchanged' or 'Modified' if any entities in the graph have conflicting key values. This may be because some entities are new and have not yet received database-generated key values. In this case use the 'Add' method or the 'Added' entity state to track the graph and then set the state of non-new entities to 'Unchanged' or 'Modified' as appropriate.

说我的实体主键重复，我检查过了，主键不重复，无论新增的还是修改的数据均不重复。请问这里我该怎么写呢？