byte relay[] = {2, 3, 4, 5, 6, 7, 8, 9, 10}; // array of pin numbers
byte pick[8] = {0, 0, 0, 0, 0, 0, 0, 0}; // eight relays
byte activated; // count activated relays

void setup() {
  randomSeed(analogRead(A0)); // pseudorandom start
  Serial.begin(115200); // start serial comm for serial monitor

  for (int i = 0; i < 8; i++) { // count 8...
    pinMode(relay[i], OUTPUT); // ... pins to configure for output
  }
}

void loop() {
  if (activated < 8) { // if all relays not selected...
    byte randomNumber = random(8); // random relay, 0 to 7
    if (pick[randomNumber] == 0) { // if relay is available...
      pick[randomNumber] = 1; // ...make this relay unavailable
      delay(random(100, 500));
      digitalWrite(relay[randomNumber], HIGH); // energize relay
      Serial.print(randomNumber); // show the relay selected
      Serial.print(" - relay ");
      Serial.println(activated);
      activated++;
    } else {
      Serial.print("repeat  - ");
      Serial.println(randomNumber);
      digitalWrite(relay[8], HIGH);
      delay(10);
      digitalWrite(relay[8], LOW);
    }
  }
}