You can already spend coins twice, it's just that one of the addresses you send the coins to ultimately won't get them. So people have to wait some time to ensu

You can already spend coins twice, it's just that one of the addresses you send the coins to ultimately won't get them. So people have to wait some time to ensure the transaction fully processes before they can give you the goods in full confidence that they have received payment. At some point, if cryptocurrencies become more established, it might be financially beneficial for one blockchain if another blockchain becomes non-viable.