I found that when a table header is too tall to fit on the page, it is carried over to the next page. BUT, then it is displayed twice, the logic being that a table row that is carried over to next page should be preceded by its table header. But since we ARE in the table header, this should not happen, of course.
Fix is easy:
Line 5193 of html2pdf.class.php should be
if (HTML2PDF::$_tables[$param['num']]['new_page'] && count(HTML2PDF::$_tables[$param['num']]['thead']['code']) && !$this->_isInThead) {
instead of
if (HTML2PDF::$_tables[$param['num']]['new_page'] && count(HTML2PDF::$_tables[$param['num']]['thead']['code'])) {