Generation of random capital letters in C# while checking to generate only one pair from each

Hi I'm trying to code basic console Pexeso game in C#. And I'm wondering how to generate one and only one pair of capital letters.

So far my code can genereate string with random capital letters but I don't know how to control it to generate only two and only two from each one.

        string chars = "AABBCCDDEEFFGGHHIIJJKKLL";
        var stringChars = new char[24];
        Random random = new Random();

        for (int i = 0; i < stringChars.Length; i++)
        {
            stringChars[i] = chars[random.Next(chars.Length)];
        }

        string finalString = new String(stringChars);

        Console.WriteLine(finalString);

Thank you very much for your help.