Magento – Why isn’t tiered pricing being applied to configurable products

I'm assuming you want to display the tiered pricing for each cart item on the cart view page, it might be worth clarifying this in your question.

It looks like you are trying to call the method getTierPrice() from the Mage_Catalog_Model_Product class instead of the Mage_Catalog_Block_Product_Abstract class.

You could create your own custom block that extends Mage_Catalog_Block_Product_Abstract but the easiest way to achieve what you want is to create a new instance of Mage_Catalog_Block_Product_View for each item.

Add this after $_item = $this->getItem(); inside checkout/cart/item/default.phtml:

$_tierPricing = $this->getLayout()->createBlock(
    'catalog/product_view',
    'product.tierprices',
    array(
        'product_id' => $_item->getProductId()
    )
);
$_tierPricing->setTemplate('catalog/product/view/tierprices.phtml');

And then where you want to put your pricing info:

<?php echo $_tierPricing->getTierPriceHtml();?>

1. On the first line we create a new product view block and pass in the products' ID. It is important this is done now during the creation phase rather after the block is instantiated to avoid errors in the _prepareLayout method.
2. Then we set the tiered pricing template, you should probably make a new template used just for the cart. Perhaps checkout/cart/item/tierprices.phtml
3. Now we can use the getTierPriceHtml() method successfully.

@pspahn's comment is correct. array_unique() will remove the duplicates. In your code example, it could be used like this:

<?php if ($_hasAssociatedProducts): ?>
    <?php $qtys = array(); 
          foreach ($_associatedProducts as $_items): ?> 
        <?php $_items->setData('tier_price',null); ?>
        <?php $_tierPrices = $this->getTierPrices($_items); ?>
            <?php foreach ($_tierPrices as $key => $value): ?>
                <?php $qtys[] = $value['price_qty']; ?>
            <?php endforeach; ?>
    <?php endforeach;
          array_unique($qtys); ?>
    <?php for ($i=0;$i<count($qtys);$i++): ?>
        <th><?php echo $qtys[$i]; ?></th>
    <?php endfor; ?>
<?php endif; ?>

This will output 2 3 5 based on your example.

Were you just wanting to display the quantities only like this?