Random numbers that don't repeat on Arduino (C# I think)

I need to code a program for school that picks two random persons of our class but only once. I think the best way to do this is to create a array with the name of everybody of our classmates and then create a random number to pick the persons from that array. My problem is that I don't know how to get a random number that don't repeat.

my code:

String student[] = {classmates};
int x;
int y;

void setup() {
  Serial.begin(9600);
  randomSeed(analogRead(A1));
  x = random(0,15);
  y = random(16,28);
  Serial.println(student[x]);
  Serial.println(student[y]);
  }


void loop() {
}

Can anybody tell me how to get every number/person only once ?

Greetings & thank's for the help