As far as I can see the Save changes and Cancel buttons (on the add/edit submission page) should get disabled when either is clicked (MDL-38555). But as far as I can see this doesn't happen with Chrome but does happen with Firefox.
Because the Save changes button isn't disabled the student can click it twice and if so gets the error:
Given the popularity of Chrome I was surprised by this but couldn't find any reports here or in the Tracker.
Please could someone with both Chrome and Firefox try this and confirm if this a general problem rather than an issue with my set-up.