How do I randomize an array without any repeating/duplicate strings in Android Studio?

I can randomize my strings that I set to each button in android but there are duplicates of the same letters. How do I do it without any duplicate letters?

    Random random = new Random();
    String[] letters = {"G","O","K","U","H","A","N","L","Z","M"};

    c1r1 = (Button) findViewById(R.id.btn1);
    c2r1 = (Button) findViewById(R.id.btn2);
    c3r1 = (Button) findViewById(R.id.btn3);
    c4r1 = (Button) findViewById(R.id.btn4);
    c5r1 = (Button) findViewById(R.id.btn5);
    c1r2 = (Button) findViewById(R.id.btn6);
    c2r2 = (Button) findViewById(R.id.btn7);
    c3r2 = (Button) findViewById(R.id.btn8);
    c4r2 = (Button) findViewById(R.id.btn9);
    c5r2 = (Button) findViewById(R.id.btn10);
    first = (Button) findViewById(R.id.first);
    second = (Button) findViewById(R.id.second);
    third = (Button) findViewById(R.id.third);
    fourth = (Button) findViewById(R.id.fourth);

    c1r1.setText("" + letters[random.nextInt(letters.length)]);
    c2r1.setText("" + letters[random.nextInt(letters.length)]);
    c3r1.setText("" + letters[random.nextInt(letters.length)]);
    c4r1.setText("" + letters[random.nextInt(letters.length)]);
    c5r1.setText("" + letters[random.nextInt(letters.length)]);
    c1r2.setText("" + letters[random.nextInt(letters.length)]);
    c2r2.setText("" + letters[random.nextInt(letters.length)]);
    c3r2.setText("" + letters[random.nextInt(letters.length)]);
    c4r2.setText("" + letters[random.nextInt(letters.length)]);
    c5r2.setText("" + letters[random.nextInt(letters.length)]);