Multiple product display nodes on (Admin) Products page

because of the last changes #1701652-8: Rebuild and retheme the product listing view,
where we built a Search box, Filter by multiple fields (new filter built within Views module, dev version), we needed the SKU field defined in the View Fields.
This fields multiplicate the node of the product display for each referenced product.

So what we need it's an aggregation for this field.