This is related to
WICKET-2 but wasn't fixed in that bug:
When you enter nothing in an input field with autocomplete and press enter, type something that doesn't give results or press escape when the autocomplete box has appeared, you have to press enter twice to submit the form.
The solution is quite simple: remove line 219 from wicket-autocomplete.js:
This variable was introduced to keep enter from submitting, but by setting it here always to 1, enter is blocked in the above cases.