You can do something like this:

let photosList = [
    './images/1.jpg',
    './images/2.jpg',
    './images/3.jpg',
    './images/4.jpg',
    './images/5.jpg',
    './images/6.jpg'
];

let photoSelected = new Array(photosList.length).fill(false);

function RandomIndex() {
    return Math.floor(Math.random() * photosList.length);
}

function showList() {
    document.querySelectorAll('.classImg').forEach(function(tag) {
        let index = RandomIndex();
        while(photoSelected[index] == true){
            index = RandomIndex();
        };
        photoSelected[index] = true;
        tag.src = photosList[index];
    })
}

showList();

Hope, it helps!!