Must use Boost theme.

Open an assignment with submissions.

Click View Submissions.

On the top of the page, Click the drop down box labeled: "Grading actions"

Click "Download all submissions"

The download box will show twice.

I traced it down to the accessibleChange custom event found here (lib\templates\url_select.mustache)

{{#js}}

        require(['jquery', 'core/custom_interaction_events'], function($, CustomEvents) {

            CustomEvents.define('#{{id}}', [CustomEvents.events.accessibleChange]);

            $('#{{id}}').on(CustomEvents.events.accessibleChange, function() {

                if (!$(this).val()) {

                    return false;

                }

                $('#{{formid}}').submit();

            });

        });

    {{/js}}

if I add $('#id').unbind("click"); before the .on() it fires only once, However, this is not a complete fix.  It requires a click outside the select box to fire.