A poorly implemented design decision then (which they turned off on AWS back in 2012 due to exactly this happening to many people). https://forums.aws.amazon.com/thread.jspa?threadID=101218

There was no way at the time for me to a) see that I had a second account associated with my email address or b) reset the password for the second account without going through support c) merging the two accounts into one even with supports help.