using System;

using System.Collections.Generic;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

            var list1 = new List<KeyValue>();

            list1.Add(new KeyValue { Key = 33, DoubleList = new List<double>() { 2.3, 2.4, 2.5 } });

            list1.Add(new KeyValue { Key = 34, DoubleList = new List<double>() { 3.3, 3.4, 3.5 } });

            list1.Add(new KeyValue { Key = 35, DoubleList = new List<double>() { 4.3, 4.4, 4.5 }});

​

            var list2 = new List<KeyValue>();

            list2.Add(new KeyValue { Key = 33, DoubleList = new List<double>() { 20.3, 20.4 } });

            list2.Add(new KeyValue { Key = 34, DoubleList = new List<double>() { 30.3, 30.4 } });

            list2.Add(new KeyValue { Key = 35, DoubleList = new List<double>() { 40.3, 40.4 }});

​

            list1.Concat(list2)

                .GroupBy(x => x.Key)

                .Select(x => new KeyValue { Key = x.Key, DoubleList = x.SelectMany(x => x.DoubleList).ToList() })

                .ToList()

                .Dump();

​

    }

}

​

public class KeyValue

{

    public int Key { get; set; }

    public List<double> DoubleList { get; set; }

}

​

​