Magento 1.7 Filter products by multiple categories

Question

I am looking for a way to filter the products being returned on a category page by the current category AND an optional sub-category. Every solution I have seen so far has b

Answer 1

About error Item (Mage_Catalog_Model_Product) with the same id "30674" already exist' in /magento/lib/Varien/Data/Collection.php:373, I found the solution:

$conditions = array();
foreach ($categoryIds as $categoryId) {
    if (is_numeric($categoryId)) {
        $conditions[] = "{{table}}.category_id = $categoryId";
    }
}
$collection->distinct(true)
    ->joinField('category_id', 'catalog/category_product', /* 'category_id' */null, 
         'product_id = entity_id', implode(" OR ", $conditions), 'inner');

1. set distinct
2. don't include the category_id field in select clause