Is it ethical..?

Seldom Killer is wrong.
you are not guilty of theft. You are not even at risk of being guilty of theft.

The Unsolicited Goods and Services act, 1971 makes it an offence for a company to demand payment for goods they have sent you that you didn't ask for.

The extra box is legally yours, and you are under no obligation to either inform them of their mistake, or return the box. Some companies claim the right, in their terms and conditions, to reclaim duplicate orders, under the Sale of Goods act 1979. This has not been tested, but the presumption is that this claimed right is invalid, and has no legal force.

http://www.onlineshoppingrights.co.uk/legally-can-i-keep-second-laptop.html

I'm not sure who the alleged expert is from this site, but the advice given seems pretty shaky. I doubt that is would stand up in a court of law. The Unsolicited Goods and Services Act (UGSA) is pretty much inapplicable for two reasons;

1) The goods were not unsolicited, there were erroneously sent twice. You could argue that the second set was unsolicited, but legally this is unlikely to hold water. Something that is unsolicited implies that is was provided through intent and not error and you'd have a hard time proving that.
2) The UGSA only covers an attempt to obtain payment for said goods or services. It doesn't cover a claim of ownership of goods that a company could entirely legally exercise.

The Theft Act, however, is very implicit on an intention to deprive the legal owner of their property or reasonable remuneration against it. Much more applicable.