I find that most digital products that you buy require creating an account because typically the account you create is how you gain access to what you're buying

I find that most digital products that you buy require creating an account because typically the account you create is how you gain access to what you're buying. I'm designing a checkout page right now where I planned to do a small 1 pager with an email address + minimal billing info required to make a transaction while minimizing CC fraud. Knowing the email (aka. account) before the checkout actually happens is essential because it would allow you to prevent someone from buying the same item twice. If you don't collect an email address pre-transaction, how would you do that for a digital item?