Duplicated Email Alias?

The email in question is one of my @hotmail.com alias (in hotmail, you can add several email aliases). Recently, I got some facebook's new account confirmation emails. Without me confirming that email, I later got a number of facebook's friend request for that account. It's seems that someone else using my email alias to open a facebook account and he can successfully confirmed his account with my email.

I keep getting his facebook activities coming to my email for a week so I decided to reset that facebook password and login to his account. I found that he is using my email alias as his primary email in his facebook account. I tried to post on his wall let him know he is using the same (or wrong) email but still no response from him so far.

Today, I got a new gmail account confirmation email originate from the same person/email.

At first, I thought he input wrong email by typos. But after seeing that his facebook account can be created as well as gmail account, I suspect that there might be something wrong with hotmail alias system. Like accidentally allow duplicated email address be created?

Thanks,

Saran