As no-one else has answered here goes.
The template that creates the product info page is tpl_product_info_display.php which is in (includes/templates/yourtemplate/templates/) Either this template is wrong or it is being loaded twice.
So, check that file to see that nothing is obviously repeated in it. This line:
<div class="centerColumn" id="productGeneral">
should only appear once.
Then when that doesn't help (most likely) have a look at main_template_vars.php (includes/modules/yourtemplate/pages/product_info/ or includes/modules/pages/product_info/). You are looking for a line that reads:
require($template->get_template_dir($tpl_page_body,DIR_WS_TEMPLATE, $current_page_base,'templates'). $tpl_page_body);
This is the line that calls the template. Again it should only appear once.
If you do not find something weird in either of these two places then there is something very strange going on.
If you haven't made any alterations to these files I would be tempted to replace them with the originals from the install ( or download another clean version of Zen and use the files from there.) Also it is always worth checking if the problem persists if you switch to classic template briefly.
Let me know....
