How To Display Random Message With No Repeats

I am trying to display a random number, but each time a button is pressed that calls the test function, i don't want to random number to be repeated. I know I have to declare 2 variables and a while loop, but i'm stuck as to what to put in the second variable and in the while loop.

var random = Math.floor(Q * Math.random());
var random2 = ??
function test () {
document.write(random);
while (random == random2) {
document.write(random);
}}