Yes, it would be theft--a shipping error does not give you the right to keep the extra computer, any more than if you had, for example, shipped a computer to a company for repairs and misaddressed the box, so that it was delivered to someone else's home, that person would have the right to keep your computer.You could require the company to pay for the cost of shipping, such as providing a FedEX or UPS label and account number--you don't have to go out of pocket for their error.