The additional field is an empty text field.
This is what appears in the HTML source:
<option value="" selected="selected" class="SelectList1">Option1</option>
<option value="ExampleText" selected="selected" class="SelectList2">Option2</option>
<option value="ExampleText2" class="SelectList1">Option3</option>
As you see, both the first options are marked as selected.
This happens when the page is loaded the first time. On subsequent loads it will correctly remember the selection from last time (if changed), with only one item selected. So how can I set this default selection manually for the first load?