Magento – Why does Magento do duplicate URLs

Take a look at how Magento generates it's sitemap.xml file.

Specifically the Mage_Sitemap_Model_Sitemap class method generateXml. For example, following snippet generates them for the categories, I've rewritten it a little to export to a flat text file.

$ioWrite = new Varien_Io_File();
$path = ''; 
$ioWrite->open(array('path' => $path));
$ioWrite->streamOpen($path.'urls.txt');

$storeId =  Mage::app()->getStore()->getId();
$date    = Mage::getSingleton('core/date')->gmtDate('Y-m-d');
$baseUrl = Mage::app()->getStore($storeId)->getBaseUrl(Mage_Core_Model_Store::URL_TYPE_LINK);

/**
 * Generate categories sitemap
 */
$changefreq = (string)Mage::getStoreConfig('sitemap/category/changefreq', $storeId);
$priority   = (string)Mage::getStoreConfig('sitemap/category/priority', $storeId);
$collection = Mage::getResourceModel('sitemap/catalog_category')->getCollection($storeId);
$categories = new Varien_Object();
$categories->setItems($collection);
Mage::dispatchEvent('sitemap_categories_generating_before', array(
    'collection' => $categories
));
foreach ($categories->getItems() as $item) {
    $ioWrite->streamWrite(htmlspecialchars($baseUrl . $item->getUrl()));
}
unset($collection);
$ioWrite->close();

Add the code from the class to your own script to generate a text file

It turns out the Redirect directive in an .htaccess file won't match against the query string, only the request uri (the part before the question mark). To match against the query string you'll need to use the RewriteCond and RewriteRule directives. More info on this here: https://simonecarletti.com/blog/2009/01/apache-query-string-redirects/

This should work in the .htaccess

RewriteEngine On

RewriteCond %{QUERY_STRING} ^main_page=product_info&products_id=331$
RewriteRule ^index\.php$ http://gearx.local/catalogsearch/result?q=sku-331 [R=301,L]

If all the urls you need to redirect are in the same format with a numeric id then these two lines should take care of them all:

RewriteCond %{QUERY_STRING} ^main_page=product_info&products_id=([0-9]+)$
RewriteRule ^index\.php$ http://gearx.local/catalogsearch/result?q=sku-%1 [R=301,L]

I'd recommend redirecting to the actual url of the product though rather than a search results page.