C# List 去重

   List<Trademark_Category_Asso> list_Group = db.Select<Trademark_Category_Asso>(exp).Skip((operation.Page - 1) * operation.Limit).Take(operation.Limit).ToList();
                List<Trademark_Category_AssoParams> list1 = new List<Trademark_Category_AssoParams>();
                                
                for (int i = 0; i < list_Group.Count; i++)
                {
                    for (int j = (i + 1); j < list_Group.Count; j++)
                    {
                        if (list_Group[j].ProductModelId == list_Group[i].ProductModelId && list_Group[j].CategoryId == list_Group[i].CategoryId)
                        {
                            list_Group.RemoveAt(j);//去除相同的项
                            i = 0;//从新开始去重，如果数组内有大量重复的项，仅一次去重不能解决问题。这样的用法会使效率慢1/3
                            j = 0;
                        }                     
                    }
                }            

                foreach (Trademark_Category_Asso a in list_Group)
                {
                    Trademark_Category_AssoParams param = new Trademark_Category_AssoParams();
                 
                        TrademarkCP cp =
                            db.Select<TrademarkCP>(s => s.CPTrademarkId == a.TrademarkCPId).FirstOrDefault();
                        PartCP1 pcp = db.Select<PartCP1>(s => s.CPNameId == a.CategoryId).FirstOrDefault();
                        PartCP cp1 = db.Select<PartCP>(s => s.CPTypeId == pcp.CPTypeID).FirstOrDefault();
                        ProductModel pm = db.Select<ProductModel>(s => s.ModelId == a.ProductModelId).FirstOrDefault();
                        param.BigCategoryId = cp1.CPTypeId;
                        param.BigCategoryName = cp1.PartTypeName;
                        param.CategoryId = pcp.CPNameId;
                        param.CategoryName = pcp.PartName;
                        param.ProductModelId = pm.ModelId;
                        param.ProductModelName =pm.ModelName;
                        param.TCAssoId = a.TCAssoId;
                        param.TCNote = a.TCNote;
                        param.TrademarkCPId = cp.CPTrademarkId;
                        param.TrademarkCPName = cp.CPTrademarkName;

                    list1.Add(param);
                }

主要代码：

  for (int i = 0; i < list_Group.Count; i++)
                {
                    for (int j = (i + 1); j < list_Group.Count; j++)
                    {
                        if (list_Group[j].ProductModelId == list_Group[i].ProductModelId && list_Group[j].CategoryId == list_Group[i].CategoryId)
                        {
                            list_Group.RemoveAt(j);//去除相同的项
                            i = 0;//从新开始去重，如果数组内有大量重复的项，仅一次去重不能解决问题。这样的用法会使效率慢1/3
                            j = 0;
                        }                     
                    }
                }            

List Group 

                var t = db.Select<Trademark_Category_Asso>().GroupBy(s => new { s.CategoryId, s.ProductModelId });
                foreach (var t1 in t)
                {
                    Console.WriteLine(t1.Key.CategoryId + " " + t1.Key.ProductModelId);
                }