Lawyers could probably spend weeks arguing about this.
The Unsolicited Goods and Services Act used to give the sender of unsolicited goods 6 months to arrange for the collection of such goods (at the sender's expense and at a time convenient to the recipient), after which they became the recipient's property. (You were also supposed to notify the sender, within 30 days of receipt, that he should arrange collection and that you'd regard the goods as yours after 6 months). The Act has now been amended and you have the right to regard any genuinely unsolicited goods as an outright gift from the moment the goods arrive.
However, lawyers could argue over whether the Hoover was 'genuinely unsolicited'. Your case would be that you only solicited the dispatch of the first Hoover, not of the second one. The sender's case would be you definitely solicited certain goods from him and that you should not benefit from a genuine error on his part.
That brings us to the provisions of the Theft Act, which makes it an offence to seek to benefit from the errors of others. (For example, if a bank employee accidentally fills a cash machine with �50 notes, instead of �20 ones, it's an offence not to return excess money which is paid out to you).
UK law requires that any business trading online must provide you with a postal address. Assuming that the trader has complied with the requirement, I suggest sending a letter by recorded delivery (and, of course, keeping a copy of the letter) stating that you expect the trader to advise you within (say) 30 days of the arrangements for collecting the Hoover. Also state that if you do not hear from the trader within that period, you will regard the Hoover as an unsolicited gift. (However, to be on the safe side, I'd hang on to it for 6 months before selling it. There could then be no argument that you'd unreasonably sought to gain from the trader's error).