Hi,
just wanted to say thank you again for your help and that now my product page works without duplicate blocks.

For everyone interested: I don't use xselling so I just commented out the line
require($template->get_template_dir('tpl_modules_xsell_products.php', DIR_WS_TEMPLATE, $current_page_base,'templates'). '/' . 'tpl_modules_xsell_products.php')

et voilá no more error log, no double product description and the whole product page is shown