I have some random string (letter-case is not matter I suppose) as an example: "aqaswasldkaslfaslyetdop".
I need to create a method which takes a string as the argument, then match all substring made from duplicate characters, delete them and return the length of the longest one substring in addition to edited input-string:
“aqaswasldkaslfaslyetdop” —>

int resultLength = GetMaxLengthOfDuplicateSubstring("aqaswasldkaslfaslyetdop", out string resultString);

Console.WriteLine($"{resultString}nTHE LENGTH OF THE LONGEST DELETED SUBSTRING: {resultLength}");

// OUTPUT:
//
// qwkfaslyetdop
// THE LENGTH OF THE LONGEST DELETED SUBSTRING: 4

I have tried to locate a set of characters which are repeat itself.

var example = "qwasldkfaslyetdop";

var matchStrings = new string[example.Length];

var distinctStrings = new List<string>();

for (int index = 0; index < example.Length; index = index + 1)
{
    matchStrings[index] = example[index].ToString();

    if (Regex.Matches(example, matchStrings[index]).Count > 1)
    {
        distinctStrings.Add(matchStrings[index]);
    }
}

distinctStrings = distinctStrings.Distinct().ToList();

foreach (string element in distinctStrings)
{
    Console.WriteLine(element);
}

But due all this permutations like "a", "s", "l", "d", "as", "sa", "asl", "sal", "sla", "lsa", "als", "las", "asld", … I just lost my initial idea how solve this problem.
I do not know why teacher from private online school made such difficult exercise, I already lost my ‘deadline’ with it. I have tried also positive lookaheads/lookbehinds, without much of success.