Various places on the Moodle system are using links (A HTML element) for actions that modify the Moodle database - add/remove data, i.e "Add blocks > HTML".

Clicking the "Add block" (once) and then "HTML block" rapidly will cause adding of several "HTML" blocks to the page.

I suggest preventing the ability for the user to click more then once on that link.

Something along the lines of:

$(".list-group-item.list-group-item-action").on("click", function(event) {
   if ($(this).hasClass('clicked'))

{       event.preventDefault();        return false;     }

else

{        $(this).addClass('clicked').trigger('click');    }

});

But, without the use of jquery, of course.