Wrong calculation of position for first item in the queue.

When you try to create second time or later userqueue, you're getting wrong position for first element. Also because of this you're getting wrong position for draggable form and then error about weight duplication.
Error occurs because of wrong SQL query:

  $query = db_select('userqueue_user', 'uu');
  $query->addExpression('MAX(position)', 'position');
  $position = $query->execute()->fetchField();

that misses condition.