SKU's have to be unique for each product.

Some store managers set their SKUs a certain way to help them easily identify products. Some managers leave the  SKU field blank when creating new products. When blank, SEO-Cart will automatically create unique SKUs by starting at 1 and counting up as new products are added to the store. As long as the SKUs are unique, they can be anything.

 - Note that SKUs DO appear on a customer's order receipt.