Random uniqe random generation

Okay so, I'm trying to create an application where when you click a button it Generates and Displays a concatenated sequence of unique Numbers between 1 and 76. I have it generating 1 -78 randomly with no dupes but I am unsure as to how I would make it so when it comes to displaying it, it displays 1 number and then increments +1 with every click.

So first click [28] second click [28, 33] and so on without duplicates. here is the code I have so far

window.onload = onclick;

function onclick() {
    document.getElementById("BtnCall").onmousedown = GenNumber;
}


function GenNumber() {
    var num = LoadNumbers(1, 76);
    num = shufflearray(num);
    for (i = 0; i < 1; i++) {
        ShowArray(num);
    }
};


function LoadNumbers(min, max) {
    var arr = [];
    for (var i = min; i <= max; i++) {
        arr.push(i);
    }
    return arr;
}

function shufflearray(input) {
    var out = [];
    while (input.length > 0) { 
        var i = Math.random() * input.length;
        var a = input.splice(i, 1);
        out.push(a);
    }

    return out;
}

function ShowArray(m) {

    for (var i = 0; i < m.length; i++) {
                document.getElementById("usednum").innerHTML += (m[i]+', ');
    }
}

Thanks for any support/help :)

needs to behave/like this https://gyazo.com/bebb7c58c402934050be8bc9be29e183

instead of this: This happens with one single click