DUPLICATE_VALUE:This price definition already exists in this price
Are you data loading duplicate price values in a PriceBook/Price Book Entry for a particular Product.
This is standard salesforce functionality preventing same Price for a product under a PriceBook.
Try cloning a PriceBook entry and saving it without chanding the Price and you will see this issue.
For each Product2/Pricebook2/CurrencyIsoCode combination, you are only allowed to load a single value. This restriction is in place so that users can have the correct price for a given product based on the price book. If you want to have different prices, you need to have different price books.