using System;

using System.Collections.Generic;

using System.Linq;

public class Program {

  static int GetMaxLengthOfDuplicateSubstring(string value, out string resultString) {

    HashSet<char> duplicates = value

      .GroupBy(c => c)

      .Where(group => group.Count() > 1)

      .Select(group => group.Key)

      .ToHashSet();

​

    resultString = string.Concat(value.Where(c => !duplicates.Contains(c)));

​

    int current = 0;

​

    return value.Select(c => duplicates.Contains(c) ? ++current : current = 0).Max();

  }

  public static void Main() {

    string value = "aqaswasldkaslfaslyetdop";  

    var length = GetMaxLengthOfDuplicateSubstring(value, out var result);

    Console.WriteLine($"{value} => {result} ({length})");  

  }

}