using System;
using System.Linq;
using System.Collections.Generic;
 
public class Test
{
	public static void Main()
	{
        var a = new List<string>{"A","A", "B", "C"};
    	var b = new List<string>{"A", "B"};
        var res  =  a.Select(e => new {Key=e, Val=1})
            .Concat(b.Select(e => new {Key=e, Val=-1}))
            .GroupBy(e => e.Key, e => e.Val)
            .SelectMany(g => Enumerable.Repeat(g.Key, Math.Max(0, g.Sum())))
            .ToList();
        foreach (var e in res) {
            Console.WriteLine(e);
        }
	}
}

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