Error: 1062 Duplicate entry '0-78' for key 1

Anybody know what this means? I get this error when trying to insert new products today. I've been using zen cart for 3 years and I've never had this error before. The products I'm adding do not have the same model number as any other products on the site, so I know that's not the issue. I checked for blank fields and there were none. Puzzled.....

1062 Duplicate entry '0-78' for key 1
in:
[insert into zen_products_to_categories (products_id, categories_id) values ('0', '78')]
If you were entering information, press the BACK button in your browser and re-check the information you had entered to be sure you left no blank fields.