I have noticed that after adding a product to cart, it disappears in list of related products. It's a default Magento feature that after adding an item to cart, it disappears under the related products of a specific product. If you want to still show that specific product even if already added it to cart You need to rewrite the block Mage_Catalog_Block_Product_List_Related and from the method _prepareData comment following line

Mage::getResourceSingleton('checkout/cart')->addExcludeProductFilter($this->_itemCollection, 
    Mage::getSingleton('checkout/session')->getQuoteId()
);

It is a good practice to not change any core file. Better to over write this file in your local folder. Create directory like

/app/code/local/Mage/Catalog/Block/Product/List and copy Related.php file in this directory.