User double clicks on "add" button

When a user double clicks the "Add" button on a Add item form, the browser usually displays a warning that the user is gonna send the same form twice. In a normal situation, the user cancels and the form is sent only once.

But supposing that the user presses enter after seeing the warning and without reading it, the damage will be done. Maybe its a careless user, maybe the user thought in a split second that the dialog box was a "confirmation" box, etc.

Is there any good strategy to avoid this kind of problem?