As I recall, this was an early decision by Amazon during the days of dialup accounts and limited email access. Two different people (eg, a couple) might share the same email address but want separate shopping accounts.

[Edit] This was covered in RISKS in 2008. See http://catless.ncl.ac.uk/Risks/25.39.html#subj12 .

Steve Loughran: Regarding the issue about Amazon allowing >1 login per e-mail address, its a historical legacy that they probably hate. Remember back in 1995 when the whole family had one compuserve or AOL e-mail address? That's when Amazon was created, and that is where they came up with the fact that an Amazon user does not have a 1:1 mapping of e-mail->userID. What they do have is a mapping of (e-mail,password)->userID; you can create two accounts with the same e-mail address, but you will get into trouble if you try and give them the same password. I'm not sure what happens, so try it and see.

The newer Amazon services, such as the Amazon Web Services, have a stricter "one e-mail address" per account rule. Clearly their support organisation has learned the error of the original design decision.

It doesn't seem possible merge multiple accounts. See this Amazon transcript for a recent example: http://www.amazon.com/forum/amazon?_encoding=UTF8&cdForum=Fx...