Random String Generator that does not repeat

I've been using this random string generator. But my problem is that this random string generator is repeating itself. I've heard of using the shuffle method but I dont know how to implement it in my code. Any help will be mostly appreciated.

private static Random random = new Random();
public static string RandomString(int length)
{
    const string chars = "0123456789";
    return new string(Enumerable.Repeat(chars, length)
      .Select(s => s[random.Next(s.Length)]).ToArray());
}

public static string RandomString2(int length)
{
    const string chars = "AB";
    return new string(Enumerable.Repeat(chars, length)
      .Select(s => s[random.Next(s.Length)]).ToArray());
}

And then in the page load i actually call my 2 methods and combine them in a textbox.

string dummy = RandomString(1);
string dummy2 = RandomString2(1);
txtTagNumber.Text = dummy2.ToString() + dummy.ToString();