I would think by product title.
Maybe have a model # field? That could the the unique field (model number)?
Or maybe have it search for similar submissions, and ask if it's a duplicate. For example, if I submit a product of a fish tank, it would kick to another screen and ask if the "new" product is one of the following already listed (results from a duplicate search) . Meaning it would post similar products and ask to make sure it's not a duplicate.
Sort of like how Digg.com works. When you submit a site, it looks for similar posts. It asks whether it's truely original or a duplicate.
I guess there are two ways to run the script. Open or closed. Open being any member can add products. Closed being only certain usergroups can add.
Having open permissions to add a product would relax the work load on the certain usergroups.
Right now review post is set that it doesn't check to make sure any portion of the products are unique. So you can post duplicate products. Even on a closed site where certain usergroups only add them, given a big amount of products you'd lose track.
So since this is about products, I would think a unique model number would be the best way. (add that field)
All I'm saying is that currently there isn't any way to prevent dupes, other than manually finding them..