I'm pretty sure Amazon allows unlimited accounts as long as you have different passwords. This is beyond bizarre. I have at least 2 accounts with the same email and the way into it is by knowing what password leads to which one. I have yet to see what happens if I try to set both accounts to the same password.
This is hard to believe. So you're saying that they hash/bcrypt (user+pass) to determine uniqueness?
However it's implemented, this is a well-known, long-standing "feature" of Amazon. I think it happened to me once when I first tried to sign up for AWS, and accidentally ended up with a second Amazon account. See http://adaptivepath.org/ideas/the-woes-of-multiple-accounts/