[1.0.0-wip] observers that create left join to a product collection throws exception.

[reillo]

duplicate issue: #268

Magento throws ErrorException when a join key name has already been defined. The BL_CustomGrid module seems called/initiated the product collection twice. This causes to some observers of "catalog_product_collection_load_before" that uses left join throws error exception.

Observer: catalog_product_collection_load_before Method Content:

$collection = $observer->getCollection();
$select = $collection->getSelect();
$select->joinLeft(array(
    'some_key' => new Zend_Db_Expr('(SELECT * FROM other_table WHERE `entity` = "product" ORDER BY `entity_id` DESC)')
    ),
    'some_key.parent_id=`e`.`entity_id`', array(
        'activeants_connector_type' => 'type',
        'timestamp' => 'timestamp',
        'message' => 'message'
    )
)->group('e.entity_id');