How to Remove Duplicate Matches in a MatchCollection

Try

Regex rx = new Regex(@"\b(?<word>\w+)\s+(\k<word>)\b", RegexOptions.Compiled);
string text = @"match match match";
MatchCollection matches = rx.Matches(text);

Linq

If you are using .Net 3.5 or greater such as 4.7, linq can be used to remove the duplicates of the match.

string data = "abc match match abc";

Console.WriteLine(string.Join(", ", 

Regex.Matches(data, @"([^\s]+)")
     .OfType<Match>()
     .Select (m => m.Groups[0].Value)
     .Distinct()

));

// Outputs abc, match

---

.Net 2 or No Linq

Place it into a hastable then extract the strings:

string data = "abc match match abc";

MatchCollection mc = Regex.Matches(data, @"[^\s]+");

Hashtable hash = new Hashtable();

foreach (Match mt in mc)
{
    string foundMatch = mt.ToString();
    if (hash.Contains(foundMatch) == false)
        hash.Add(foundMatch, string.Empty);

}

// Outputs abc and match.
foreach (DictionaryEntry element in hash)
    Console.WriteLine (element.Key);