using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main(string[] args)

    {

        List<string> symbolList = new List<string>() { "1", "1", "3", "2", "2", "2" };

        List<string> valueList = new List<string>() { "a1", "b1", "c3", "a2", "b2", "c2" };

        var items = symbolList.Zip(valueList, (symbol, value) => new {symbol, value})

            .GroupBy(pair => pair.symbol, pair => pair.value)

            .OrderBy(group => group.Key) //optional, only if you want the output to be 1,2,3

            .Select(group => String.Join("|", (new[] {group.Key}).Concat(group)));

        var result = String.Join(",", items);

        Console.WriteLine("result: " + result);

    }

}