Product List - Sometimes show duplicate products

If the product names aren't the same, and you only are sorting by name, the order of the products with the same name is undefined.

A simple solution might be to enable the flat product catalog of Magento under

System > Config > Catalog > Frontend

After reindexing, Magento will use the flat table for the store view on product listing pages on the frontend.
It might be that solves the issue already for you.

If it doesn't solve your problem, try adding another sorting criteria, for example the entity_id.

You can do that using an event observer for the catalog_product_collection_load_before event. ($observer->getCollection()->setOrder('entity_id', 'ASC');).

After that, the order will be defined, even if the product names are the same.

Actually the SKU is not 100% guarantee of product uniqueness. Merging DBs or some import methods may still leave you with duplicate SKUs.

To find out if your products are duppies or not refer to products ID. You can either output ID in catalog/product/list.phtml template or just check ID in product's URL.