using System;

using System.Collections.Generic;

using System.Linq;

​

​

public class Program

{

    public static void Main()

    {

        List<string> head = new List<string>(){ "AA", "BB", "CC", "AA" };

        List<string> tail = new List<string>(){ "CC", "AA", "DD", "EE", "FF" };

        // make lists equal length

​

        foreach(var x in head.ConcatOverlap(tail)){

            Console.WriteLine(x);

        }

    }

​

​

}

​

public static class Ext{

    public static IEnumerable<T> ConcatOverlap<T>(this IEnumerable<T> head, IEnumerable<T> tail){

        var skip = 0;

        var hLen = head.Count();

        while(head.Skip(skip).Zip(tail, (a,b) => a.Equals(b)).Any(x => !x) && skip < hLen){

            skip++;

        }

        return head.Take(skip).Concat(tail);

    }

}