Stackoverflow question where the asker and the first answer are the same guy with the same username but different accounts.
How is that possible?
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Usernames are not required to be unique by the system.
As @waiwai933 says usernames are not unique on SO, so it's entirely possible for two accounts to have the same name. However, if someone has created two accounts deliberately then that would be sockpuppetry and is not allowed.
However, in this case it looks like either an unregistered user or someone forgot which e-mail address they used to register. Flag one of the posts for moderator attention explaining about the possible duplicate user account.
It's more obvious if the gravatar image is the same.