[Dilettante_Pro]

Вариант nerv без for

alert(promptNumbers(3));

/**
 * @param {Number} count
 * @returns {Array<Number>}
 */
function promptNumbers(count) {
    let arr = new Array(count),
         i    = 0;

        while (i < count) {
            n = Number(prompt('Enter a number'));
            if (isNaN(n)) {
                alert(`It is not a number!`);
                continue;
            }
            if (contains(arr, n)) {
                alert(`The number "${n}" already exists!`);
                continue;
            }
            arr[i] = n;
            i++;
        }
    return arr;
}


function contains(arr, needle) {
    return arr.indexOf(needle) !== -1;
}