Ajax button strange behaviour when clicked twice
Problem solved!
The button must be of type "submit" instead of "button"! So the correct code is:

$form['auctionpersons']['add_person_btn'] = array(
  '#type' =&gt; 'submit',
  '#value' =&gt; 'Add person',
  '#description' =&gt; 'Click button to add person',
  '#ajax' =&gt; array(
      'callback' =&gt; 'add_auction_person_ajax_callback',
      'effect' =&gt; 'fade',
  ),
);


Thx for viewing this question :-)