using System;

using System.IO;

using System.Text.RegularExpressions;

using System.Linq;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var filePaths = new string[] {

                @"\\server\TestDB-1.bak",

                @"\\server\TestDB_Customer (1).bak",

                @"\\server\TestDB-3.bak",

                @"\\server\TestDB_Customer_01.bak", // single

                @"\\server\TestDB_Customer_02.bak", // single

                @"\\server\TestDB_Customer_01_1.bak",

                @"\\server\TestDB_Customer_01_1.bak", // duplicate

                @"\\server\TestDB-2.bak",

                @"\\server\TestDB_Customer (2).bak",

                @"\\server\TestDB_Customer_01_2.bak",

                @"\\server\TestDB_Customer_03_1.bak",

                @"\\server\TestDB_Customer_03_2.bak",

                @"\\server\TestDB1.bak",

                @"\\server\TestDB2.bak",

                @"\\server\TestDB3.bak"

            };

        var fileNames = filePaths.Select(p => Path.GetFileNameWithoutExtension(p));

        dd(fileNames).Dump();

    }

    public static IEnumerable<IEnumerable<string>> dd(IEnumerable<string> fileNames) {

        var longestFileName = fileNames.OrderByDescending(f => f.Length).First();

        var result = new List<IEnumerable<string>>();

        for(int i = 1; i < longestFileName.Length; i++) 

        {

            var searchString = longestFileName.Substring(0, i);

            var grouped = fileNames.GroupBy(f => f.StartsWith(searchString));

            if (grouped.Count() > 1)

            {

                foreach (var item in grouped.ToArray()) {

                    result.AddRange(dd(item));

                }

            }

            else if (grouped.Count() == fileNames.Count()) {

                return new List<IEnumerable<string>>() { fileNames };

            }

        }

        return result;

    }

}