using System;
using System.Linq;
using System.Collections.Generic;
using System.IO;

public class Test
{
        public static string AddIncrementFileNameSuffix(string path, IList<string> paths, string separator)
        {
            int num = 0;
            string[] tokens = path.Split(new[] { separator }, StringSplitOptions.None);
            string strNum = tokens.Last();
            int.TryParse(strNum, out num);
            string fileName = path;
            string dir = Path.GetDirectoryName(path);
            string ext = Path.GetExtension(path);

            var dups = paths.Where(n => n.Equals(fileName, StringComparison.OrdinalIgnoreCase));
            while (dups.Any())
            {
                fileName = Path.Combine(dir, tokens.First() + separator + ++num + ext);
            }

            paths.Add(fileName);
            return fileName;
        }
    
	public static void Main()
	{
        var SourceFiles = new List<String>() { @"C:\Temp\test_1", @"C:\Temp\test_2" };
        var lsNameList = new List<String>() { @"C:\Temp\test_1", @"C:\Temp\test_2", @"C:\Temp\test_3" };


        IEnumerable<String> newPaths = SourceFiles
                .Select(fn => AddIncrementFileNameSuffix(fn, lsNameList,"_"));

        Console.Write(string.Join(Environment.NewLine, newPaths.ToArray()));
	}
}