I recently managed to do that as well. This becomes very interesting if you try to recover a password, because apparently the password recovery only recovers the last account.
Eventually I got out of that by changing the mail of whatever account I could log into, using gmails feature of ignoring everything between the first + and an @ in the address. But most annoying, now my aws login is something silly like [email protected].
Maybe I could change that back, but I'm too scared to touch that.
It probably should, but I somehow ended up with two Amazon accounts (retail, not AWS) with the same email address. Changing the password I use changes which account I log in to.
Presumably it's a bug, but it's certainly confusing.