For all intents and purposes an email address is a unique identifier. This is the status quo that the rest of the internet maintains.

I've got two Amazon accounts under the same email address that happen to have two different passwords. My order history is more or less split evenly between the two since either password mnemonic works. It has caused such frustration that I even stopped buying digital goods on their platform and switched to using Google Play. (It's not the only reason for my decision, but it certainly factored in.)