How to stop a random string generator from repeating a string consecutively?

Is there a way for this random string generator to stop repeating a string consecutively?

public class ButtonText : MonoBehaviour
{
public Text TextField;

public void PickRandomFromList()
{
    string[] letters = new string[] {"A", "B", "C", "D", "E", "F"};
    string randomName = letters[Random.Range(0, letters.Length)];
    TextField.text = randomName;
}

If you wish to avoid the function picking the same letter twice in a row, just store the previous value:

 private string lastLetter="";
 public void PickRandomFromList()
 {
     string[] letters = new string[] {"A", "B", "C", "D", "E", "F"};
     string randomName;
     do {
         randomName = letters[Random.Range(0, letters.Length)];
     } while (randomName == lastLetter);
     lastLetter = randomName;
     TextField.text = randomName;
 }