Here's the main post about this on MSO. The idea is that people should be allowed to use their real names as their usernames, and just as multiple people have the same real name, multiple people need to be allowed to have the same username.
If the new Alex was intentionally trying to impersonate you, I'd take action. Given that he's not, there are plenty of ways to disambiguate, starting with your Gravatar image and rep score. One can tell at a glance that the other Alex isn't you.
If you really want to ensure that you won't get inadvertently impersonated, you can change your username to something that no one else would use unless they're intentionally copying yours. Or, you can pick any point on the uniqueness spectrum between a non-rare first name on one hand and a string of random characters on the other.
