https://medium.com/@nikus/magento-1-x-301-redirect-from-product-url-duplicates-1175ff633116

magento-301-redirect-for-products.php

<?php
public function viewAction()
{
    // Get initial data from request
    $categoryId = (int) $this->getRequest()->getParam('category', false);
    $productId  = (int) $this->getRequest()->getParam('id');
    $specifyOptions = $this->getRequest()->getParam('options');

    /////////////////////////////////////// START WITH 301 REDIRECT
    $product = Mage::getModel('catalog/product')->load($productId);
    $currentURL = Mage::helper('core/url')->getCurrentUrl();
    $getParams = '';
    $params = $this->getRequest()->getParams();
    if (isset($params['id'])) {
        unset($params['id']);
    }
    if ($params) {
        $getParams = '?' . http_build_query($params);
    }
    if ($currentURL != $product->getProductUrl() . $getParams) {
        $this->getResponse()->setRedirect($product->getProductUrl() . $getParams, 301)->sendResponse();
    }
    /////////////////////////////////////// END WITH 301 REDIRECT

    // Prepare helper and params
    $viewHelper = Mage::helper('catalog/product_view');

    $params = new Varien_Object();
    $params->setCategoryId($categoryId);
    $params->setSpecifyOptions($specifyOptions);

    // Render page
    try {
        $viewHelper->prepareAndRender($productId, $this, $params);
    } catch (Exception $e) {
        if ($e->getCode() == $viewHelper->ERR_NO_PRODUCT_LOADED) {
            if (isset($_GET['store'])  && !$this->getResponse()->isRedirect()) {
                $this->_redirect('');
            } elseif (!$this->getResponse()->isRedirect()) {
                $this->_forward('noRoute');
            }
        } else {
            Mage::logException($e);
            $this->_forward('noRoute');
        }
    }
}