List<StoreDetailDto> items = new List<StoreDetailDto>();

        items.Add(new StoreDetailDto { StoreNumber = 2, WarehouseNumber = 4201, IsResolved = false, StoreName = "StoreEx", WarehouseName = "WarehouseEx1" });
        items.Add(new StoreDetailDto { StoreNumber = 3, WarehouseNumber = 4201, IsResolved = false , StoreName = "StoreEx", WarehouseName = "WarehouseEx1"});
        items.Add(new StoreDetailDto { StoreNumber = 6, WarehouseNumber = 4201, IsResolved = false, StoreName = "StoreEx" , WarehouseName = "WarehouseEx1"});
        items.Add(new StoreDetailDto { StoreNumber = 7, WarehouseNumber = 4201, IsResolved = false , StoreName = "StoreEx", WarehouseName = "WarehouseEx1"});
        items.Add(new StoreDetailDto { StoreNumber = 9, WarehouseNumber = 4201, IsResolved = false , StoreName = "StoreEx", WarehouseName = "WarehouseEx1"});

        items.Add(new StoreDetailDto { StoreNumber = 6, WarehouseNumber = 4202, IsResolved = false, StoreName = "StoreEx", WarehouseName = "WarehouseEx1"});
        items.Add(new StoreDetailDto { StoreNumber = 7, WarehouseNumber = 4201, IsResolved = false, StoreName = "StoreEx" , WarehouseName = "WarehouseEx1"});
        items.Add(new StoreDetailDto { StoreNumber = 9, WarehouseNumber = 4203, IsResolved = false, StoreName = "StoreEx", WarehouseName = "WarehouseEx1" });
        items.Add(new StoreDetailDto { StoreNumber = 9, WarehouseNumber = 4207, IsResolved = false , StoreName = "StoreEx", WarehouseName = "WarehouseEx1"});

var overlapDupStores = items.GroupBy(u => u.StoreNumber, u =>
        {
            return u;
        }, (key, g) => g.ToList()).ToList().Where(cnt => cnt.Count() > 1);


        foreach (var dpovl in overlapDupStores)
        {
            var stores = dpovl.Where(g => g.IsResolved != true).GroupBy(u => new { u.StoreNumber, u.WarehouseNumber }).ToList();

            if (stores.Count() > 1)
            {
                response.OverlappingStores.AddRange(stores.SelectMany(gr => gr).ToList());
            }
        }

var warehouseCountsByStore =
            from item in items
            group item by new { item.StoreNumber, item.WarehouseNumber } into store
            where store.Count() != 0
            select new
            {
                store.Key.StoreNumber,
                store.Key.WarehouseNumber,
                Count = store.Count()
            };

    var storesWithDuplicateWarehouses =
        from warehouse in warehouseCountsByStore
        where warehouse.Count != 1
        select new { warehouse.StoreNumber };

    var storesWithMultipleWarehouse =
        from item in items
        group item by new { item.StoreNumber } into store
        where store.Count() > 1
              && storesWithDuplicateWarehouses.Any(x => x.StoreNumber != store.Key.StoreNumber)
        select new
        {
            store.Key.StoreNumber
        };

    Console.WriteLine("Duplicates");
    foreach (var store in storesWithDuplicateWarehouses)
        Console.WriteLine(store.StoreNumber);

    Console.WriteLine("Overlap");
    foreach (var store in storesWithMultipleWarehouse)
        Console.WriteLine(store.StoreNumber);