1. A new record for a product that already exists
The clearest example is worth stating plainly. You buy the same lamp from a second supplier and set it up under a new internal code, so your warehouse can tell the two consignments apart. You then create a new catalog record to match your new code.
Nothing about the lamp changed. Your code is an inventory fact about your business; the catalog record is a claim about the product. A second code for the same lamp is normal operations, and a second record for it is a duplicate. The same reasoning decides whether two items may share a parent: see variations abuse.