Fastest way to implement Duplicate Character Removal in String (C#)

Question

In C#, what is the fastest way to detect duplicate characters in a String and remove them (removal including 1st instance of the duplicated character)?

Answer 1

This one should be pretty quick (and it preserves the original order):

public static string RemoveDuplicates(string source)
{
    HashSet found = new HashSet();
    HashSet dupes = new HashSet();

    foreach (char c in source)
    {
        if (!found.Add(c))
        {
            dupes.Add(c);
        }
    }

    StringBuilder sb = new StringBuilder(source.Length);
    foreach (char c in source)
    {
        if (!dupes.Contains(c))
        {
            sb.Append(c);
        }
    }
    return sb.ToString();
}