To fix duplicate content issues on product pages in OScommerce open up your product_info.php file

place this code

Code:
<link rel="canonical" href="http://www.example.com/product_info.php/products_id/<?=$HTTP_GET_VARS['products_id'];?>" />
directly above your </head> tag so it looks like:

Code:
<link rel="canonical" href="http://www.example.com/product_info.php/products_id/<?=$HTTP_GET_VARS['products_id'];?>" />
</head>
Assuming you use the SE friendly URLs, if not, change the url to the format you use.

All done. Easy, why didn't I do this earlier?